Sorry for the delay in response, if any of you are still around.
It's been rather hectic here!
The two problem children were with me before my friend moved in. When I got them, they were NOT litter box trained which I'd never had to do before. I got them there though. When my friend moved in with her cats, they started being problem children again. It's almost certainly related to the new cats!
I have a two floor house. The new cats (the ones that came with my friend) spend 90% of their time on the top floor and there are two large boxes up there. There's one in her bedroom and one in mine (the cats spend most of the days in my room and then she takes them to bed with her).
My cats spend 90% of their time on the lower floor and there are two boxes on that floor as well. There's one in the foyer at one end of the house and one in a closet under the stairs at the other end. There's nothing that should have scared them near these boxes.
I did quarantine the problem children together. They're from the same litter and are usually pretty good with each other. That said, the boy from this pair gets along very, very well with the new cats. The girl does NOT but she doesn't really get along with any of the other cats except her brother.
I've worked out something that works quite well for the wall peer. I put the box in a large lid and line the lid with poise pads. These are absorbent enough to pick it up easily enough and I just replace them each day. This works very well actually! I still have to clean the lid that leans against the wall but that's not a big deal if I clean it every evening.
We spent over a month introducing them. Before she moved in, we swapped blankets. Then she brought them over in their crate. Once she moved in, we kept them in her room for a couple of weeks but encouraged play between them by threading string under the door.
The dog was kept in one room for quite a few months after my friend moved in. Once she seemed less likely to chase the cats (she was never mean - she just wanted to smell them and see what they were), we started letting her out on a leash. The cats still aren't very happy with her but she's definitely more afraid of them than they of her!
I might try the Kids 'n Pets Jassy recommended or another enzymatic I've heard about. The homemade just hasn't seemed to cut through it very well - even when used with a steam cleaner. The vents, at this point, are just blocked.
It really comes down to these two who weren't very well litter trained in the first place. Even once I got them trained, there were some issues here and there with them. It's just that now they're more likely to pee somewhere OTHER than the litter box. Maybe I'll try a reintroduction with these two.
It's funny because one of my cats is REALLY unhappy with the new cats but she still uses the litter box. We just have to keep an eye on her so she doesn't go after them - especially since they can't defend themselves!