I'm so sad to hear your news, crisper - I know myself how horrible it is.
We had something similar happen many many years back, when diagnostics weren't nearly as good as they are now. One of my then cats was clearly unwell - losing weight, scrawny - but the vets just didn't know what it was. It turned out to be kidney failure (which they can treat well now, but 30 years ago they couldn't.....) - but the vets clearly were out of their comfort zone. They kept him in - I rang up to see how he was, only to be told that he'd died in the night. They didn't ring me.....
The trouble with cats, as I found recently with Black Spot, is that no matter how ill they are, they don't tell you until they are really in a bad way - by which time it may be too late :-(
Please don't feel you left her to suffer - but did you explain to the cattery about her state of health when you left her there? They could genuinely have thought otherwise that the heat was affecting her (as it does).
You could complain (as I could have done), but I was so devastated that I didn't, and not venting my spleen wouldn't change things. I just didn't go back to that vets with my remaining cat at that time. Cat 2 needed a lot of comforting, he knew his brother was gone - cats know these things....