Congrats, interesting experiment. One thing I've discovered in my Danish AAR is that swapping territories with the AI can lead to settlement rebellion; i.e. offer a Muslim or Orthodox nation a province from your country in exchange for a province from theirs (For example, I gave the Byz Venice in exchange for Thessalonica I think). You know how to manage religion difference, and because you initiated the trade you can be ready for it, while they're rapidly sucked into a rebellion, at which point you could reconquer the settlement. Having your own spies and priests already present doesn't hurt either.

I'm still trying to figure out if garrison size in a traded city is purely a function of population or if nearby garrisons are taken into account, because if a city lands a massive garrison it can be much harder to bump off. It's not a bad idea to buy up all the decent mercs in the area too.