Hi,
First off, I would recommend adding litter pans. A good rule of thumb is one more pan than the number of cats you have. Even if you have to line them all up in a row I would do so. Some cats will suddenly decide they don't want to urinate and defecate in the same pan, or that they don't want to go in a pan that one of the other cats just went it. Having more pans gives them more appropriate options. This works for a lot of cats, but of course not all.
Otherwise, it sounds like you are doing everything right. Here is a link to a page that may have some additional ideas for you:
http://www.peteducation.com/article.cfm?c=1+2174&aid=158
One other thing that works for many cats that spray is a hormone injection. Its called Depo-provera, and you will have to discuss it with your veterinarian. Some cats quit spraying altogether with one injection, others have to get them routinely. There is a slight risk of side effects though we've not had any problems when using it at the veterinary clinic I work at.
If you have further questions let me know. If this answer is helpful please click accept. Thank you.
Lynn
A prescription drug is the most likely thing to work for your kitty in my opinion. You've already tried the other things that occasionally work (Feliway, spray bottles, etc.). Other than adding litter pans there really isn't anything else to try. Spraying is a very difficult problem to deal with, and is probably the reason this kitty was in a shelter to begin with. It is a learned behavior, and once they start doing it, its hard to break. There are other prescription medications that can be tried first before Depo-provera though. (I just mentioned it first because I personally know of good results with it.) Anti-anxiety medications also work for many cats- Prozac, Buspar, and Clomicalm are all some that can be used. You may want to try one of these first as the possible side effects from these are less severe. I would definitely discuss this with your vet as the effectiveness of these medications is fairly high.
Veterinary Assistant
worked at a vet clinic for 15 yrs, BA in biology, have had cats most of my life