I just found out someone has a LP going for Civup and Gem + Infoaddict. Enjoy- http://www.youtube.com/watch?v=2BCiwKHD0oM
Keep in mind that the civ changes weren't finished. Spain was supposed to get a "get faith on conquest" UA + UU spread their religion to conquered cities remodel which would make them play differently. (Though I personally would have thought combat bonus vs CS + double NW yields would be more fun, but that suits another civ as well)
And yes, the peace talks for the AI function over your military strength (unit count). It doesn't take into account your past success and your territory which is a major flaw in civ 5. It's a vanilla problem though. What it basically means that you are better off just building a few more units to get them to peace talks (if you don't want to conquer them). It's a pity.
I agree, but I don't think it's that easy to change otherwise there would be a mod improving those things which to my knowledge there isn't.
(I'd argue that the whole unit mechanism of civ5 is screwed up since it doesn't actually matter for war success how many units you have, but how many you can produce. This way, gameplay doesn't reward long-plan thinking (in regard of units). But that's another topic.)
The manual trigger you're suggesting sounds promising (if it's doable), but I fear it may also lead to unexpected results. From my observations, there's often the case that the upper level AI has changed its goals for the games but the lower level tactical AI can't make peace since the conditions are not favourable to them (army > human army; even though they haven't made progress for the last 50 turns). As soon as my power grows (often through simply upgrading the units) and we can make peace, their stance changes to friendly, since their new game goal is aligned with my playstyle... (or warmongers like warmongers which I now am since I took one of their citis...). It's the basic diplomacy that's screwed up since vanilla.
I hope that civ6 can go away from these things (since AI will never be up to par) and more into the roleplaying side of things. That's just easier... (but now I am really off topic)