Mattress update
Okay, so when I contacted the mattress company about returning it, they offered to send us a 3 inch latex mattress topper that's $400 on Amazon for $200. They told me that in 99.9% of cases this solves the firmness problem, and if it doesn't they would be willing to pay the return shipping for me to send it back.
Since this topper is made of latex (same material as the mattress) I could only assume I wouldn't have the overheating problem I have with memory foam.
It just arrived via FedEx. It's super nice, covered in the same material as the mattress, so it basically creates a pillow top effect. Obviously, I'm going to have to sleep on it for a night to see how it is, but I am hopeful that this will be the solution.
My only real remaining concern is long-term happiness (just based on a few reviews, but I suspect I could find those kind of reviews for almost any mattress) but there's really nothing I can do about that. And the whole shebang still came in under $1k, so it's not like we spent a small fortune on it. If we have to replace it sooner than we hope, it wouldn't be the end of the world.
Fingers crossed!