Are you sure the AC was working in your partner's room? If they were asleep I'm assuming you didn't go in there to check, but moving you into a studio for a night may not have actually solved the problem if your partner woke up sweltering a few minutes later.

That being said, your proposed solution seems like a good one for all involved, and it's a shame you had to fight so hard with the GM to get it to happen. The only thing I can think of, which doesn't excuse her behavior but could explain it, would be if the AC went out in a whole block of rooms and she's suddenly fearing all the guests who are going to want another room to move to (which, if she was really at 96% full would become 100% full really fast).

The other thing, like you said, would have been to open the window and turn off the AC so it wouldn't be blowing the warm air in. Given the 50s/60s temps, that should have cooled things down pretty quickly.