So a few questions for you -
Did these "problem children" do this before your friend moved in in their old home? If not it sounds like they are trying to assert their dominance over the others. Also if not could you use the exact same litter your friend had used in her old home?
You said you had four large boxes but never said where they are located. Are they close together? Some cats don't like having another box near them while they are using one.
Are they near a noisy appliance (dryer, washing machine, etc) which could have turned on when they were in the box(es) and scared them. So that solution is to move the boxes to another area.
If you are using storage boxes then they likely don't have lids so I won't ask if they are enclosed. Some cats just don't like enclosed boxes. Then again others do so if that is what they were used to in their old home..........
Could you try the Swheat Scoop in one, the cat attract in another, clay in yet another and pine in the last? That gives all cats the choice. Or do two Swheat Scoops, one Dr E's cat attract, and one clay.
You said you quarantined the two problem children. Did you put them together to do so or did your put them each in their own space? Putting them together in another room is only going to exacerbate the problem. As soon as one sprays or pees the other is going to follow suit. Try separating them in their own room and see if that makes a difference. Make sure those rooms have been totally cleaned of lingering pee though or you may never overcome the issue. Use a black light to check over the whole room. It will pick up any lingering spots of urine.
For your wall peer - can you use a taller box cut down on the outside so he can get in easily but the tall side toward the wall where he always pees? That way the pee is being redirected back into the box, not your wall.
How were the cats originally introduced? Were they just thrown in together and said hey, get used to each other or did you do a proper separation, scent exchange, sight but no contact, exchange of rooms, and then slow exposure to each other? It's difficult at the best of times to introduce one cat to others. When you are trying to do three extra and a dog to boot - trouble!!! You may find it beneficial to start afresh and try doing it all over again.
I use the homemade recipe for my cats. I clean the spot(s) first with a good cleaner, allow to dry, and then liberally soak the area a few times to ensure the smell is gone.
Assumptions do nothing but make an ass out of u and me.
We can stick our heads in the sand for only so long before it starts choking us. Face it folks. The pet population is bad ALL OVER THE WORLD!