I too have peaked at the surrendering AI but have found more often that their situation isn't terrible; they just need to hunker down and play defense for a bit. But then again ,I play for fun and not for competition or speed.
Problem is that hunkering down doesn't actually help all that much in these situations, and in some ways actually makes things worse. Spending large sums on starbases and other defenses leaves you with little for your economy and military, meaning if you were already behind in these categories you're only going to fall further behind.
It's particularly annoying when the AI does this in a 1v1 situation. In a FFA there is at least the possibility that the strongest players might cripple each other and let the little guy catch up, but in a 1v1 situation it basically amounts to digging your own grave since there is no plausible way you will ever turn things around. To use a sports analogy, you can't play defense and ride out the clock if your team is behind!
That said, were you able to find a way to "cheese" it by building say, 1000 scouts or whatever?
Scouts are actually pretty expensive in terms of their credit cost, and are only notable for not costing metal or crystal. If you have reasonable resource income, Corvettes and Light Frigates are actually cheaper
The bottom line, though, is that if you're in a position where you can actually get away with building 1000 scouts then your opponent is definitely
Edit: That said, in the past surrendered players would generally park their fleets (in my experience) but now it seems liek they will still jump to their other holdings to defend. Up to a point. THIS may be dependant on the quality of AI. BUt this spawns to other questions for you:
I'd guess this is something that was done for multiplayer, since a fleet sitting in the middle of nowhere can easily become titan-chow.
1. Can you delay surrender behavior by improving an AI's difficulty level after a game has started/mid-game?
The higher difficulty level will increase its income and allow it to build more units. It stands to reason that it should be less prone to surrender, yes.
2. IS there a utility that can un-surrender a player? I think it could be neat if somehow surrendered AIs can spawn into a new player or something, this increasing the incentive to wipe them off the map. Nuke them form orbit, just to be sure.
Not that I'm aware of.