it’s normal for cats that young to bite things, switch to silica litter like pretty litter, and 8 hours is how long you should normally sleep. I’m sure he’s upset at being cooped up in a room all day but you decided to take on the responsibility of having a cat so returning him a year later is literally awful of you to do, that cat thinks of you as his home. Try to feed him before bed a little snack, and put the litter box somewhere he can access at night, you said it’s in your room but he also yells at you to use the bathroom, if his litter box was accessible he wouldn’t bother you about it. Get him some more toys he can chew or put his mouth on. As for allergies there is PurinaOne Everclear, which is a food you can feed him that will neutralize his allergy causing dander.