I wrote to the store owner about Thing 2 and the condition he was in. For him to pas so soon after I got him means he *had* to have been in that condition for a long time. I'll be keeping a close eye on the bettas there from now on.
Fleur has recovered remarkably, and suspiciously, quickly. In a few hours in water with stress coat and epsom salt, she went from floating at the top of her tank, having to fight to swim down, and not being able to get even halfway down her tank before tiring and floating back to the top, to being perfectly normal and healthy! I'm going to keep her in the epsom salt for another day, just in case, especially since I haven't seen her poop. The bloat I noticed has gone down completely, though, so I'm hoping she just needed a little TLC :)