This won't be of much help to you, but I'm having a similar issue. Practically ALL of my townie deaths are caused by "overheating". Even killed off an important NPC (that I really liked too, rip Morgyn the Mage- currently trying to bring him back without cheating) :C
All to say, just about an hour ago my main sim just RANDOMLY got the lethal heat moodlet (again), but this time, he didn't survive. Neither did the butler.
it was WINTER. Previously the AC was on COLDEST and one of my other sim was complaining that it was like a morgue so I turned it off and then my other boy died of heat almost right after. Almost like he needs to be freezing otherwise he can't survive lol. I didn't save my game there because I was too bummed about it.
What I did to fix it was:
>buy all the heat resistant perks (I was lucky I had saved up),
>Put my sim in warm weather clothing to cool him down,
>saved,
>closed the entire game
>repaired my game, including delete the exceptioncrash (or lastexception, can't remember fully the name) files and the local thumbcache and that fixed it for me.
All in that order because, even after buying the perks ,the moodlet it was not going away so I had to fix the game twice, because fixing it the first time without buying the perks did nothing either. I also wanted to avoid cheating as much as possible because it defeats the purpose of playing to me (unless obviously the ONLY fix is by cheating, then I would have resorted to that last).
I didn't want to have the fireproof perk because I do like the challenge of real weather, but I also preferred that he didn't die because I'd lose everything in his inventory, his jobs and whatnot.. And this way of dying just wasn't right. Had I been greedy and actually kept him in scorching weather I would have accepted it, but this one did NOT make sense and nothing I did before in-game prevented it (including taking cold showers, turning the AC back to cold, making him swim in almost freezing water - he even shuddered of cold while dying a few minutes later overheating).
I've had issued with the lethal heat moodlet appearing on my playable sims before as well, but always managed to avoid it (while other townies just dropped like flies), until this time. I also find it odd that it is NEVER gradual. It's always, all of a sudden your Sim is dying. No warning no nothing..
Just noticed your post was in 2018- whoops. Sad to see that problem seems like it's still around ☹️
