The further I went through this thread the more I just couldn't imagine this happening. I hope the little tort gets better and no more damage happens. I would still suggest getting rid of the heating pad as it could have been the culprit and change the substrate at least monthly. Good luck and I am sorry you had to share such sad news.
So sorry for the little guy. Hope it can get around okay.
I would suspect the heat too. Seems too much of a coincidence that both feet fell off, and at the same time.
Was this at a joint? Could there have been a bilateral injury kind of crushing or breaking both joints at once?