If OR isn't relevant to the discussion, than neither is any talk of settling GP's in a specialist economy, which, of course, is an assinine contention. If one is relying upon superior production to gain an edge, than OR is eminently relevant. If you cannot concede that, well then...
Going early metal casting forgoes access to caste, the great library, national epic, and monarchy, and thus has a direct impact on your ability to attain early GPP beyond 2 library scientists/city.
Going MC vs Monarchy (or any other tech) has the same requirements in terms of OR (you still have to research or trade for mono). In other words, the
use of OR does not vary between alternatives. It especially has nothing to do with stopping barb galleys. Teching MC to the exclusion of those "useless techs", however, has very real implications on GPP.
Only if you want to waste tons of inefficient hammers building large money eating garrisons. I often skip monarchy until very late. I don't want to spend an inordinate amount of time building mass units that take forever to build and that I'm just going to mothball. As to forges/granaries, I have no set rule regarding these. I often do build forges first, and those forge first cities, in the long run, often do grow faster than my granary first cities. After the forge is built, the granary and work boats come much quicker. Besides, maybe I'd prefer a barracks and a couple of troops first. Or walls. It depends. Nothing irritates me more than a city that is emphasizing food so much that it cannot build anything--including the granary that is supposed to be making it grow twice as fast.
Maybe I want to go forge, monument, library, theater in a border city that is fighting for culture. Building a granary, contrary to your assertion, won't make this happen faster.
Oh, wow. There's a good reason most good players (not just me, but people far better than me) go granaries and prioritize either monarchy, drama, or pyramids heavily. If you're struggling with this, I suggest very strongly that you look up why that is. Assuming you're going to ultimately work more than 1-3 tiles, the granary will overtake other options ridiculously quickly. Are you really willing to ignore the body of evidence on the forum?
Warriors hammers are a very efficient source of

and can allow growth...FAST growth...like 20 pop capitols at 100 AD with some resource trades also. You're not going to get that with metal casting, usually.
Faster troops, faster buildings, leftover hammer multiplication, all for faster total growth. MC is expensive for a reason. That reason being that it is a powerful tech with game-altering abilities. precisely because production is paramount in Civ.

. You're blinded by a 25% multiplier. This paragraph I'm quoting up here is a non-argument. Here, I can do it too:
Faster troops, faster buildings, leftover hammers, all for faster total growth. pottery/monarchy is expensive for a reason. That reason being that they are powerful techs with game-altering abilities. precisely because production is paramount in Civ.
Yay. Continuing on to useful discussion:
Now this is nonsensical. How else do you defeat barb galleys then? Pray? Rename a workboat to "Mr. Rabbit" and have the barb galleys chase him to your neighbor's territory? Bribe a neighboring civ to declare war on the barbarian nation? I have many games where the protection of seafood is paramount. You really didn't address this point very well (or at all).
I guess all those immortal isolated start (LHC) games I post where I build 0-2 galleys and get 0 seafood pillaged aren't good enough.
But I'll say it in case I didn't address it in this thread directly (pretty sure I have): barb galleys can only "see" 7 tiles into your culture, and will not enter to pillage seafood further in AND
Land units and work boats can spawn bust to prevent barb galleys from spawning in areas where they'd be an issue.
Edit: No, I didn't say it in this thread because Ghpstage did so I didn't see the need:
There are 2 game rules that govern barb Galleys that can be used against them;
They can only move into your culture if they can 'see' a pillageable resource, the sight range is 7 tiles (I think ). So by selectively settling and possibly even avoiding improving one specific seafood resources, you can prevent galleys entering your borders
The other is that barbs cannot spawn in your sight area, and cannot spawn within 2 tiles of any unit (regardless of sight). This one is standard 'fogbusting' useful on any map where barbs will be an issue.
Apparently that went over a few heads though, if what I said remains "nonsense" despite being *proven*.
Combining the above, I've yet to see anything but spotty island archipelago (and only some of them) that required more than 2 galley cover from one side...and even then only until a city can be planted that blocks entry.
Since I play most of my games now with tech trading off, alphabet is a useless tech. I skip it until the industrial age. Even with tech trading on, however, alphabet gives less than aesthetics (simply bcz AI's don't emphasize it, and therefore want it). because aesthetics is better trade bait. I would prefer a mathematics, currency path myself. More hammers from my trees (which my forges and OR like) and another large trade route for my monolithic cities. But if we're talking trade value, MC has more than any other tech during that era. All of the other civs really want this one, and will trade their cottages away for it.
How am I supposed to know you play with trades off? However with them off MC for triremes hurts you even more unless you have some good early

resources and can use the colossus.
Alphabet opens up building research, funny you don't mention that.
Deity players, playing deity, go alpha before aesthetics on occasion. It seems to work fine for them. Note that alpha also unlocks currency, and math is a high-priority AI tech that you can trade for if they're on.
There isn't such the wide disparity in tech value that you intimate. All of the techs pretty much cost what their value really is, so simply make use of their value following your discovery of them.
The disparity is there. Tech path choices can mean winning vs losing. The AI tends to prioritize MC and will usually have it in the BC's on emperor+ (on normal settings, anyway). You better hope you get a lot of trade or resource value for MC, because if you don't you're in cap trouble.
What other options? Using a tech suboptimally rather? See adjacent repsonse above.
Options such as choosing to research other tech over suboptimal MC just to protect seafood and build an early forge in 1-3 cities.
Look, I'm not saying MC and triremes are bad, just that going that route in the BCs to stop barb galleys is going to be a weak choice in A LOT of games.