i hate my cat

TLDR; my "roommate" stole my cat by overfeeding and spoiling him, they are now bonded, and my cat has become nothing but an annoyance to me.

so, i've had this guy for about six years now, since he was only six months old. the first 2-3 years were easy because i adored him. he is extremely affectionate, silly, and full of personality. he has always been a bit of a shit...mainly because he knocks things off of high surfaces for entertainment. this is NOT the problem, it's always been tolerable because i loved him so much. he also has a medical condition that requires expensive medicine 2x a day, every day, for the rest of his life. i will add that i have a second cat, who is a girl. so i'll call them boy cat and girl cat.

somewhere within the 2nd year of our life together, i had to move in with my partner and their grandparent grandparent had an elderly boy cat at the time who has since passed away.

in the process of trying to acclimate all the cats to each other and to grandparent, well...grandparent basically stole my boy cat. they are fully bonded now. grandparent fed boy cat multiple times every single day to the point of obesity. we repeatedly asked grandparent to stop feeding boy cat, but they would just keep doing it. we had to stop feeding boy cat ENTIRELY because we just wanted him to be healthy, which did work. he lost the weight and now maintains it exclusively on the diet fed by grandparent. but boy cat slowly stopped spending time with me, and eventually stopped being affectionate with me altogether. it's about 4 years into this situation and boy cat and i no longer like each other at all.

his behavior with grandparent has gotten worse and worse over the years because grandparent literally calls boy cat "the boss" and willfully lets boy cat control their entire life, and they will not listen to me when i tell them to ignore boy cat's demands. they're so far into this shit at this point, i think it would take twice as long to undo the amount of damage that's been done to boy cat's behavior.

example of demanding behavior: boy cat meows at grandparent's door, gets let in. boy cat immediately turns around, scratches the carpet, gets let out. turns around to meow again, grandparent gives him treats and leaves door open. boy cat goes into room, starts knocking things over. grandparent gives more treats outside of room, shuts door. boy cat meows. cycle repeats literally ALL DAY EVERY DAY. the meowing in the hallway wakes me up at 4am regularly.

when boy cat isn't meowing, grandparent is nonstop talking to him in this high pitched baby voice. it was cute at first, but it's nonfuckingstop, we have thin walls, and it is just so droning and irritating at this point. hearing them encourage this behavior and repeat the same phrases over. and over. and over. every. single. day. to a cat that doesn't understand what the hell they're saying. sometimes, grandparent will announce that they're putting boy cat in whatever room we're in, because boy cat is bothering grandparent. (well, no shit he's bothering you. you turned him into a monster. don't make that my responsibility when i warned you repeatedly for four years. he's your cat now.)

nowadays, all boy cat does is piss me off. he comes into my room just to scratch up my expensive chair with his claws that he doesn't let me trim (nail trims used to be a bonding experience, he loved being held and touched and had no problems with it before we came here), knock my breakable trinkets around, and attack my precious girl cat. he wakes me up constantly despite not even being in my room. he has to be shut in a separate room while we eat at the dining table because he will just wander around looking for ways to get a reaction out of us. all i do is pay for and administer his medicine, clean his litter box, and half-heartedly play with him since grandparent doesn't.

i cannot fucking stand him anymore. he's no longer my cat by bond, and i still have to deal with all of the shitty parts of him. i'm exhausted. i would never hurt this cat or any animal, obviously, and i don't even have the option of rehoming him because nobody is going to want him and grandparent would be devastated. i just don't know what to do at this point.

please don't recommend a cat behaviorist. i already know how to handle these things with force-free training and behavior modification...the problem is that grandparent ruins any behavior modification plans without fail.

has anyone else had to deal with a similar situation before? what did you do? how do i handle this? i feel defeated, but i'm just posting here in case anyone has some creative ideas besides "give grandparent the cat and move out."

PLEASE NOTE: grandparent is fully cognizant. this is not a memory care patient. they don't even use a walking stick/support yet, and are 100% of sound mind. they are an absolutely wonderful human being, but they just don't listen or care what i say when it comes to this cat. they think they are providing "love" to the cat, when really they just have zero boundaries and are letting the cat control them. we always say that grandparent is boy cat's jester.