I haven't had time to read all the posts. Just want to say a few things, first and foremost, it's always easy to arm chair quarterback and say "I would never do....this or that". Until you walk in that person's shoes, please don't judge. Never say never because Karma will come and bite you at some point in time. Just saying.
2nd, poster is asking for advice, suggestions and ideas. Not judgements or criticisms. Sometimes people just need to express their frustrations about their situation and sometimes find something that works for them. Some of the best problem solutions come from collaborating and thinking outside the box.
Like I said, haven't read all the posts so I may be asking questions that have already been asked...
How many litterboxes do you have? Ideally, there should be one litter box per cat.
Is your litter box(es) covered? Open litter boxes work best. Placement of litter boxes is also crucial. They should not be placed in a busy area with lots of traffic. Do not place litter boxes in the same area with other litter boxes. Have them in different, low traffic areas of the house/apt. I find that corners work best, it gives them a bit of privacy and 'safety' with two walls, yet still open enough for easy access. Do not place litter boxes in dark areas, and use a night light near the box if the room(s) are unlit at night, or place them under tables.
Have a deep litter box and use TONS of litter. Scoop boxes several times a day. Cats are fussy about cleanliness and do not like messy, smelly, dirty litter. They like to bury their stuff. The more litter, the better for the cat(s). Do not believe that more litter means you don't have to clean it out several times a day.
Try different types of litter. Clumping litters are what we humans like, but your cat(s) may not like it. Avoid dusty litter, it makes them sneeze. I advise not trying to save money by buying the cheap litter in bulk. With litter, I do believe you get what you pay for.
You said your cats have been vet checked. Did your vet check for diabetes or low thyroid levels? With both diabetes and low thyroid levels the cats will drink lots of water and pee often. The pee output is ALOT! Medication can help with these issues but it takes time to get the correct dosage right for the cat. Each cat is different.
Sometimes it's just one cat who causes the behavioral type peeing issues. Changing the group dynamics may help. If looking to rehome one or more of your cats, be HONEST with the person considering taking in the cat about the peeing issues so there are no surprises; someone who is fully informed of issues and is willing to take the cat (s) on will less likely rehome the cat. Some cats prefer to be single pets, some like having other cats around. Some cats get overly stressed with sharing the home with other non cat pets; birds, dogs, rodents, etc.
Praying that you are able to resolve these issues, or if you have to rehome, praying you are able to find the perfect forever home (s).