Have you been able to isolate the cat that is peeing? Just hearing your story, I would bet that the one that hides by the vanity is being stalked by the alpha female and she is afraid to use the litter box, so therefore pees where she feels safe. Or perhaps there is a power struggle with the boys and they are marking to claim territory.
Having 14 in my house, I can tell you that any change in routine will upset my guys - new addition, change in my schedule, moving things around the house. How stable is your routine?
Are they choosing the same area over and over again? If so, you may not be getting the scent out of that area, or you may need to put a litter box in that location. Mine used the corner in my rec room until I put a litter box there and haven't messed since.
Do they have enough territory in your house? I've added a lot of vertical space with tall cat condos so that everyone has their place in the house.
My new puppies added chaos for a long time. The cats would wander into their kennels are mark their bedding. A lot of puppy training later to calm them down around the cats stopped that problem.
In a multi-cat household, there isn't a single solution to problem pee'ers. You need to study the situation to see what triggers it, who does it, when they do it, where they do it and why they do it. You take action appropriately. With 7 cats you need to be constantly vigilent around them or it will get out of hand quickly.