Next I contact my good buds the Sumerians who are way ahead of me in tech and I pay them 1000 gpt for a variety of IA techs, their 2000 gold and an alliance against England. I then kill off England on the same turn, change my taxmen back, and put luxuries back at 20 percent.
I say that's fine by me. You aren't getting any gpt from the AI that they no longer have*, and that's the distinction. As you said, you are not getting money that no one has. This whole hullaballoo if firaxis didn't make it so that a deal is immutalbe due to circumstances. IOW, if the Sumerians are paying you 500gpt and are losing a war and due to losses can no longer afford it then you should only get the 455 or 310 or whatever for the turn and it should go back up if it improves or lower accordingly. This probably would have been too hard to program, I know I certainly couldn't I'm a player of games not a maker of them, but that sounds better and these would never be possible to begin with.
*BTW Does this give you a rep hit? I don't remember, but that's not a rules question, but I was wondering if there actually is any in game downside to this. Seems like there should be, but we know there are problems to work around.
One more thing, I think some people have the word "retroactively" confused with arbitrarily, as if some thing is retroactive would be done with out any consideration to the players but on a whimsical basis. There is nothing of the sort here from any of the staff past and present.