"As to Number 2, I don't consider that an advantage. As long as I mind my manners and don't bust my rep, I don't need peaceful relations with the AI to continue trading. I can still buy and sell luxes, resources, techs and alliances."
Well maybe... but do you do so as effectively? Do you get as much gold as you could and luxuries and/or tech for as cheap as you could?
I am not the best trader on these boards by a long shot, but I do believe that I trade effectively.
Doesn't AI attitude go down when you accidently auto-raze cities?
Attitude is different from reputation. After the Ancient Age, I'm generally surprised if the AI is anything other than furious with me. But that doesn't stop them from buying my techs.
. . . . Still... you have at least one fewer trading partner. So, you won't get as much traded goods as you could have... at least in terms of the tech pace. Your war with say the Romans will slow them down tech-wise... especially if you crush them. Also, there comes another problem I'd say.
One less trading partner and one less enemy. You're right, though. An AI's tech pace really, really slows down when you eliminate them.

Maybe crushing the Romans means that I won't get their gold. It also means that nobody else will, either.
I've never been quite clear on the effect of eliminating civs on tech costs, but I know it has an impact. I
think it lowers the cost, but may be wrong.
. . . . If you want your cities to become metros with either C3C or C4C . . . .
And
that is one of the linchpins of the city spacing debate. "If you want your cities to become metros . . . " If I want my cities to become metros, I need to allow for extra tiles for these metros to work, meaning that tiles go unworked until Sanitation (or Shake's). If I want my cities to become metros, then I have to build hospitals, and pay upkeep. One lucky city might get Shakespeare's Theater. if I want my cities to become metros, I have to be ready to clean up population pollution. Some of my cities will get factories, so I'll still have to clean up pollution from them, admittedly.
. . . . you war with someone in the ancient or middle ages, then you need to build units and barracks and maybe even raise your luxury slider . . . . or you need specialists in your cities which slow down production and commerce. This doesn't work well with metros, since for a metro you work as better served to build as much infrastructure as possible . . . . Since I usually aim for metros instead of cities, I'd rather maintain peaceful relations with the AI until at least Education, so 2 works out as an advantage. Building barracks and units, and infrastracture at the same time either needs handled very, very carefully to work well or results in inefficiencies, I'd think. . . . .
I'm sure this is all true, but it's also (as far as AI can tell) based on the premise that you're going to want metros. I don't see any need for them. Does a size-20 metro produce more gold, shields and beakers than a size-12 city on the same terrain? Sure. But it only starts producing more after it hits size 13.
"However, the fastest way to build it is with an MGL."
I forgot about that. Still, you need two great leaders spawned from combat (one for the army, one for the Military Academy).
That's just one MGL more than I would need to build the MA by hand. Not that big of a deal. Truth be told, I usually build the MA by hand, anyway.
I kind of guess you play as militaristic tribes a lot. It also sounds like you either use or would want lots of artillery for cheap elite wins. I almost never play militaristic civs.
I do like militaristic civs. I enjoy an early archer-rush and cheap raxes don't hurt my feelings, either. With that said, many players who are far better than I really don't value militaristic very highly.
"Once I hit the 90% cieling, where every new town is going to be 90% corrupt, there is no downside to putting down more towns."
Do you consider what sort of help a courthouse and police station might provide? Some of those cities might actually produce more science with a courthouse and police station (at least at certain times in the game if not overall) than a specialist farm or perhaps two specialist farms. Alright... from your comments you do.
I almost never build Police Stations, but I do build courthouses. It depends on how much difference the CH is going to make. I use CA2 to judge how much good the CH will do.