Mark it's hard to cope with all the time. I had a peeing prob with one of my Burmese Bow many years ago and i never realised why he was doing it. Every day he would wee up all the skirting boards in every room and my moggey Ben became nervious and and hiding. Back then i don't know they didnt like each other and it was because of that. When my Bow passed on to the bridge, Ben became a totally different cat and got on great with my other Burmese Arnie.
I might have replied before but if i was in your position now and not wanting to rehome Willow, i woiud def get in a cat behaviourist hun. I am pretty sure the prob won't mend itself and the behaviourist do deal with this all the time as it's the most common problem they deal with. If they give you a solution that works with keeping all the gang together i bet you wished you did got them in before and with all the money on the sprays, it would prob cover their fee? . If they say that it the prob is here to stay if Willow, Kyle and clapton were to stay, then at least you know what the solution would be and have that decsion to make or not. It might be for the best for the cats and you and your partner. I feel for you and the cats cause you all are upset over it.