I've never thought of grabbing Mandate of Heaven early as being particularly attractive, though I'll admit that it's not something I've experimented with much. I'd think that in most situations increasing your faith from shrines and temples would get you a lot further ahead than cutting your costs by 20% (especially if you're playing wide and have already taken the Piety opener), but maybe in some cases where you're getting most of your faith from pantheons or city states it would be an attractive choice.
From what I can tell, it's essential for strategies that either involve Piety starter or Liberty Collective Rule into Piety. Pagodas essentially end up functioning like Monuments, and the only way you can get that ball rolling is if your first two or three Pagodas are 20% off. Once you've brought your faith generation up to speed, that -20% isn't as vital anymore, and yeah, the +1 faith from shrines and temples does give you more faith later. The reason +1 faith from shrines and temples is not chosen first is because you'd need to get 40 bonus faith from that policy to be on equal footing with Mandate of Heaven Pagodas, and that doesn't really end up happening in time (you'll maybe get an extra 6-7 faith per turn when you need those Pagodas up ASAP for happiness to keep your ICS going). The policy also comes in handy with Jesuit Education, since lategame faith costs are so high anyway, -20% faith off of three research labs is equivalent to one extra GP from faith. But yeah, if Mandate of Heaven were unlocked after Organized Religion, the discount wouldn't come quick enough to matter as much; that policy can be quite useful, but only if rushed and only if you need Pagoda happiness ASAP (eg. you're Liberty in an area with only 1 or 2 unique luxuries).
How prominent is warblocking in multiplayer? There are definitely lategame situations where you'd want to do it to lock in city states, but in the early and mid game it doesn't really seem worth it. You're also blocking yourself from your opponents' city states (and if you're doing this because they went patronage, they probably have more than you), and you're forgoing the possibility of trade routes with them. This seems like a pretty big sacrifice to set back only one of your opponents.
Warblocking becomes more and more prominent the higher the other players' skill level: it's not because more skilled players are more aggressive, it's because more skilled players are usually the ones able to adjust their strategies the most efficiently to get the most out of warblocking. Even people who tend to turtle and try to zoom up the tech tree to XCOMs and Stealth Bombers will do it. Warblocking the person on the other side of the map is standard practice, especially if their policy trees indicate that they're going Patronage (note: if you only select Patronage opener, it doesn't show up when other people look at your social policies, so it's often a good idea to only get the opener so that you don't reveal your hand to the other players). The way it's usually done is by paying a city-state for influence (or donating a unit) in order to ally it, then quickly DoW the original CS ally to make sure they can't get it back; such a move can be quite devastating for low happiness empires reliant on the luxuries provided by CS allies and/or mercantile CS's.
Unlike in SP, you're also not the only one going for CS quests, so unintentional warblocking is much more prominent: basically ever CS ends up being allied to someone, and if that person ever declares war on you for any reason, you lose the friendly benefit from the CS (assuming they don't cleanse your religion from their allied CS to remove Papal Primacy).
International trade routes are an extremely bad idea in multiplayer: even if you don't intend to war your opponent, your opponent can just war you out of nowhere, and you'll instantly lose your trade units. Even if neither of you war each other, international trade routes are much more easily pillaged because they usually cover much larger distances than internal routes. Plus, food and hammers from internal trade routes are usually much more useful than gold (main exception being of you're going for a gold purchasing strategy and don't need hammers as much): 6 food per turn in a city is more valuable than 21 gold per turn from an international trade route, not to mention 6 hammers per turn in a city that's rushing to build an important wonder. The science you'll get from trade routes is only useful in Ancient and Classical, where that +1 or +2 science can mean a 10% beaker boost if you're only generating 10-20 beakers per turn. Religious pressure from international routes is pointless in most circumstances: if they don't have a religion, missionaries are a much faster conversion option, and if they do have a religion, chances are that they will be reinforcing their own religion's pressure with internal trade routes.
The only thing a player will lose by warblocking is the possibility to trade luxuries and research agreements. Oh yeah, and you don't want to warblock someone who's got an army large enough to win a war, but that's a known given. You can also think of the hammers the warblocked civ has to dedicate to military units as an added benefit, since the aggressor can time their warblock, while the victim has to react to the unexpected turn of events.
Space Procurements charges 3000 gold (thereabout) for each part. That makes for roughly 2-3 XCOMs per SS part. That point aside, SV and DV aren't really related. While you could theoretically take a DV after playing scientifically, it's much easier to pay about 18000 gold and win scientifically, using your city production to make units should they truly be necessary. Also, once you choose our Ideology, you simply cannot change o another one unless there is pressure for it. Say I pick Freedom aiming for an SV, but then I change my mind and go for a DV. DVs are much harder without Autocracy to back them, especially in the late-late game (XCOMs) or with Gandhi. (Curse him and his nukes.) So really, it's hard to simply switch Victory types after Ideologies.
SV is too slow and too easy to counter in Multiplayer: you need two non-essential techs, Particle Physics and Advanced Ballistics, which have their own non-essential requirements. Building Apollo Program also means painting a target on your head, not to mention sacrificing a load of hammers that could be used to make bombers to upgrade into Stealth Bombers the instant Stealth is researched. If you wait until you've got all space techs unlocked to finish Apollo Program, you've probably spent the previous 50 or so turns building bombers and useful buildings non-stop, which means you could just as well use those military units to win a domination victory. Basically, if you can win SV in multiplayer, chances are you can win Domination victory as well, and domination victory snowballs in a way that SV does not (the extra cities you get from defeating another player give you more hammers and more tech to keep snowballing).
Oh yeah, and space parts can be nuked. It doesn't matter how fast you can build space units, your opponents will be sitting there with their fingers on the trigger to nuke your space parts. They won't nuke XCOMs because by the time they see your XCOMs, your XCOMs will already be in their territory.
Above all, you have to remember that multiplayer games are usually quite fast once people get Plastics: even if you get free tenets, between having to fill out Rationalism for Great Scientists and getting key tenets (like Barracks happiness or +XP from Autocracy, +Science from factories from Order, or +15% military unit production for Freedom), you often won't get the chance to reach level 3 tenets quickly enough. Picking up Space Procurements early means delaying filling out Rationalism, which means delaying your Great Scientists, which means your opponents will have key techs before you do.
Unless you're aiming for a cultural victory, you won't be picking ideologies for victory types either: sure, Autocracy's Gunboat Diplomacy is useful, but not when you're already warring the CS's allies, so you're only getting influence from your own allies, so Clausewitz's Legacy is almost always better, especially because you'll often be picking up your level 3 tenet so late that Clausewitz's Legacy will last you for the rest of the game. You'll be picking ideologies based on free tenets, possible happiness, possible hammers, and possible science, with Total War's +15 XP functioning as an extra bonus to ensure all units you create come out triple promoted. Autocracy isn't always necessarily picked because people want to go DomV or DV: it's often picked because of the ludicrous amount of happiness you can get from it, especially with Prora. Freedom isn't picked for SV, it's picked for State of Liberty's ludicrous, empire-wide hammer boost. Order isn't picked specifically for SV or DomV: it's picked for its quick happiness (Socialist Realism), the fact that Worker's Faculties generates more science than any other tenet in any ideology if you don't have a lot of academies, and Five-Year Plan's excellent hammer boost.
If you've only played Deity Singleplayer, I highly recommend you give multiplayer a shot: unless you're playing with newbies, chances are you'll be surprised how much of what you know from singleplayer Civ5 is a facade when faced with people who can and will war you at any time to ensure that you don't win.
On the topic of Reformation beliefs and other things that are useless for Deity...
Reformation beliefs are usually not worth the effort, even on lower difficulties. This owes itself to AI stupidity: even Babs goes Piety quite a bit. In general, you aren't guaranteed a good Reformation even on King. In MP, you might not benefit very much off a Reformation, because other players don't care one bit if you can convert barbs with missionaries or purchase Public Schools with faith, because at the point where you've gone down Piety enough for a Reformation belief, they could have gone down Honor and built a force enough to obliterate you. For the most part, other things not viable on Deity aren't viable in MP either. Just as Exploration is terrible in Deity SP, it can even be worse in MP. Most other policies/tenets mentioned on his thread aren't viable in any circumstance, SP or MP.
You've never tried Liberty on multiplayer, I see. Reformation beliefs are important, nay, vital to any Liberty player in order to catch up to Tradition players' lategame science. This is because Liberty players need to make full use of the fact that Faith is the only yield in the game that rewards wide play, so they need to convert their faith to science if they want to keep up with Tradition players' huge, centralized science. Jesuit Education and Glory of God are the two important beliefs: the former's benefit is obvious, the latter's benefit comes from being able to purchase Great Scientists without having to fill out Rationalism. This means you not only make up for getting policies slower as a wide player, but you also free up the policy points you'd have spent on Sovereignty and Scientific Revolution to get an extra tenet or two instead. As an added bonus, Glory to God also lets you purchase Great Engineers to make sure you're not always beaten to Prora, Statue of Liberty, Hubble, or any other important lategame wonder by a Tradition player. Sacred Sites is also a neat little gimmick that is often hard to pull off because everyone will know what you're trying the instant you select it, but I thought it's worth a mention. If you can keep a barbarian camp alive in your backdoor, barbarian conversion is a great way to speed up your timing pushes if you know you can't beat the other players at the science game. All in all, Reformation beliefs are essential in MP.
Exploration is also quite useful in MP, but only the opener and the first two tenets. The opener is useful because it lets you DoW someone and take their coastal cities without any warning due to how ludicrous your naval units' movement points are, especially if you are England and/or have Great Lighthouse as well. +3 hammers on coastal cities is essential to any coastal Liberty strategy, as is +Happiness from naval buildings.
Going down on Honor is pointless because the bonuses you get are not worth the increased cost of all social policies thereafter: getting opener and warrior code, maybe Discipline too, is all you can afford, and really need, before Rationalism unlocks and you need to dedicate your policy points to filling it and ideologies out ASAP.
As for Consulates war-blocking, that strategy shouldn't work on anyone who's gone down Patronage, because Philanthropy+Consulates means cheap allies, meaning you can maintain moee than just a few. Also, while you're in Patronage, you can use Scholasticism to get the tech edge, MP or SP, which would make war-blocking even less effective than it is in the first place.
But it does work. In fact, it works extremely well. Really, you should try playing against veteran MP players (I'm sad to admit that I don't consider myself one).
Cheap allies don't mean jack when people aren't spending gold to buy city-states in the first place unless they plan on warblocking immediately afterwards. Remember, you're not the only one going for CS quests in MP. Plus, while you've unlocked Consulates, the other players have gone down commerce and are making enough extra gold for your cheaper allies not to matter: you might be able to spend 250 less gold to maintain ally status, but they're making that much extra gold from Commerce over the amount of time it would take for influence to decay below ally status. It's why Patronage opener's slower decay is vastly more powerful than Consulates' resting influence boost.
Scholasticism's "tech edge" is nothing: 25% of what city-states produce is huge on Deity because CS's get the same beaker boost as Deity AIs, but in multiplayer, it's maybe 5-8% at best, and it's not even consistent. You'll probably get more science out of Mercantilism's beaker boost.
/turn is almost never attractive.
or an election away. Artistic Genius' bonus comes not just in the artist, but in the ability to get easier theming bonuses or golden ages (especially if you're Persia or Brazil, or just won the World Congress and have a GW waiting 
). And Fine Arts just kinda depends on the game. I've had it pay huge dividends personally.
/turn.
from luxuries, which is always the last commerce policy and feels like the real finisher). Buying merchants probably isnt the best use of faith in most situations, but at least it doesnt impact your production of scientists and engineers.
and +4 
