Oh no, poor you. I understand your frustration with cats peeing on your furniture, Jasper does this, not as often as your kitty though, only when there is a male stray in the area.
Have you noticed any cats wondering around outside? Could they be un-neutered males? If so, he maybe marking his territory???
I am surprised your vet has not recommended the transdermal gel. I use some on Sweet Pea for her aggression against Puddles as she is IMPOSSIBLE to pill, and is very easy to do. I am currently using amitriptyline. It does come with regular vet monitoring/bloodwork though, so it is not cheap.
http://www.marvistavet.com/html/body_amitriptyline.html
Good luck.