Tyler update
I forgot to post about it here but Tyler’s problem turned out to be much simpler: he had fleas. A really bad case of them, which should have been obvious to me, but I am apparently oblivious to the difference between “he is unable to clean himself” and “the crusty junk all over his hindquarters is flea poop.” And the pain was just because he was super overwhelmed by, y'know, having bugs crawling all over him and biting him.
He is now on flea medication, as is Fiona (even though she never got infested, better safe than sorry), and he’s back to his old cuddly self. I also thorogughly combed him and disposed of so many dead fleas, and have done so much laundry.
I think I’m going to try once again to keep Tyler and Fiona indoors all the time from now on. Wish me luck.