This is a method used to help socialize and introduce ferals. I've never used it, so I don't know what to recommend. I don't really see how it helps solve the dominance problem. I think the main ingredient there is going to be time, and positive reinforcement of all other behavior from the family.
I'm glad you're getting Stanley to the vet, because that's always the first thing to do when a cat pees outside of the box. The stress may have resulted in a UTI. But it may well be a stress problem.
I'd go for the Feliway, and I'd consider purchasing Rescue Remedy flower essences as well. Both can be purchased here: http://www.catfaeries.com
. I'd also continue scent swapping (wipe Stanley all over with a towel, put that down with treats on it for Marley and vice versa). I'd give each kitty an extra 10 - 15 minutes of alone play time every day.
Since he finds your room to be his "safe" room, I'd put food, water and a litter box out for him there and give him extra love and attention.
Also, make sure you use an enzyme cleaner to clean the sheets, blankets and mattress and any place else he goes (we always did the wash with it alone and then washed them again with regular soap). I'd get a black light to go through the room and the house to make sure he hasn't gone any place else. When Spook had a peeing problem, we SOAKED the mattress where she went, covered it with thick towels, and made the bed. It took like a week to dry, so we swapped the thick towles out every two days. We had to soak the mattress just once each time it happened, but the couch took three applications of enzyme cleaner. We used GALLONS of it. We purchased this product: http://www.nokout.com
, though Anti-Icky Poo also seems to have great results, and it's also available at http://www.catfaeries.com
Poor Stanley is scared. Patience, time, and attention are going to be your best friends for helping him. With time, the scent swapping may also help. And providing positive reinforcement to Marley any time he walks by and ignores Stanley or any time Marley just sniffs at Stanley without batting at him should also result in lots of "good kitty" praise from the family.
Cats are not necessarily social animals, and returning Stanley for a new cat may not necessarily result in a cat that Marley is going to like.
We have six rescued ferals. After a couple of years, it was so apparent Lazlo, our first rescue, would have been just fine as an alone kitty. Now - the next five and all the fosters inbetween weren't for Lazlo's benefit. They were homeless animals that needed to be cared for, so as we told each of them when we brought in a new cat, they were homeless once and just had to deal with it. But my point is, Marley may not need a buddy, and I think that instead of "returning" Stanely, you should work with him to help him feel loved and safe in your home - or, at the very least, the room he's picked out to feel safe in.