I would start with the already suspected offenders, Jim and Sterling - and mainly Sterling because you've already had Jim tested if I remember. Since Sterling seems to have just started spraying, I would start the separation with him. However, if it's not him, all bets are off and I think your solution of separating them and getting a sample from each one is the best way to do it.

Good luck and hang in there! I'm sure you'll figure this out and get it taken care of because you're one of the best kitty moms I know. And remember, the kitties pick up on your stress, so you have to remain calm so you don't upset them and make matters worse. It might be a long week, but you'll make it through it and all the kitties will be fine, so take a deep breath (glad you got the panic attack over with ) and you'll be fine.

Oh, and also, just really watch the cats for awhile. I know when Tubby has his UTI he was acting very strange and it wasn't hard for me to catch on that something was wrong. I know a lot of cats don't show it like this, but if you watch them very closely, you might pick up on little clues here and there that might indicate who's having the problem.