Well, to give y'all an update, I left the snail alone overnight and sure enough, this morning, "he" was dead. So there was no need to euthanize "him."
Yes, Astreas have a bad habit of falling, but if you don't right them, they usually cannot do it themselves and they die. For whatever reason, this particular snail fell several times over the past few days and yet continued to climb the glass.
All the other snails, hermits, and fish are fine and water params are normal.
I appreciate the *helpful* feedback. As much as I agree a $3 snail is not worth agonizing over, I also believe that if I can do something more humane with the same effort, I am going to take that route. And besides, I didn't want snail guts splattering on me :lol: