I used to do a lot of foster/rescue work with abandoned cats/kittens. In a regular cat family, when one kitten bites too hard, the other one screams and then stops playing. If you do that - not a rage scream, but a squall like "OW! that hurt" and then walk away & don't engage for 5 minutes or so. Go in the bathroom w/ door closed, or whatever. If it's a poor bite inhibition (not knowing how hard to bite before it hurts) that'll cure it. On the other hand, if it's boredom/loneliness you can cure by playing w/ kitty more. Best thing to do is get a wand toy (stick, string, feather/etc on end) and draw it over the ground so kitty can chase/pounce. Do this until kitty gets bored, and then feed kitty. Mimics their natural behavior - they hunt, pounce, then eat. Makes them feel more calm & gets out the energy, etc. Don't use hands for toys until kitty is really good at bite inhibition & careful with your skin. I have cured many a bitey kitty w/ the above, including adults who were ingrained in the behavior. nose flicking will just confuse them. Spray will stop them from biting, but will also stop them from approaching and interfere with your bond and make for an insecure and confused kitty. insecure and confused kitties tend to act out in other ways, like potty outside the box, scratching, etc.

Can't offer advice on $ or health situation bc I stink at money management and am in chronically poor health, but wanted to give my animal behaviorist expertise, lol. Sorry if you weren't looking for advice, I know unsolicited advice can be annoying. I hope you are able to get through these issues and find a way to thrive!