I know you didn't like the Cat Attract litter because it didn't clump well - so what about using regular clumping litter with the Cat Attract additive? That seemed to help a while back....
And I have an interesting thought, though I don't know if it is either practical or whether or not it will work. That said..... they have cat doors that are activated by a collar the cat wears. Any way to put a cat door in your bedroom door so Frankie can come and go but Wickett can't?
Also, someone else with a litter box problem was just asking about a site hissy referred them to. Because it's a behaviorist, maybe it's worth the $80 to give it a shot? Here's a link: http://www.catpsych.com/352.html
You can try the retraining. We put Spooky in our guest bedroom when we'd gotten to our wit's end. I spent all day in there with her (I could work from home) - and she still started stress over-grooming, though others have had success by doing this.
We ended up using Elavil (amitryptaline). I know you've avoided using meds all these years, but keep it in the back of your mind. Many hate this idea - but it really helped us, and it's helped a lot of others. I know Spooky benefited from it. It is NOT effective as an anti-anxiety medication. Most people doctors actually prescribe it to help people get sleep. But our vet swears by it for litter box problems - and it worked for us. It causes cats to retain their urine. Spooky would do one huge pee a day - always in the box. It started working like almost RIGHT away - even though it does take two weeks to get fully into kitty's system. She did sleep a lot for the first few weeks. But for us, it gave everyone a much needed break. After that, she was back to her playful, loving self - just happier, no longer over-grooming, and no longer peeing out side the box. We kept her on it for 4 - 5 months, and weaned her off of it over a few weeks.
I'm really not sure why everyone is so against medications for kitties in situations like this. I agree - everything else should be tried first. But when you've done that - where do you turn? I know plenty of people on mood altering drugs - they're going to be on them the rest of their lives, and they're happier - more stable - people for it. I don't know why there is such a huge stigma against this option when it comes to pets.
I just know this has been going on for years with Frankie now. For Spooky, separating her to retrain her just stressed her even further. We had to do something to break the cycle. And it worked.
Just something to keep in mind.