Mark sorry to hear of your troubles
Deciding to rehome a cat you have had for years is actually a selfless act, sometimes its in the best interest of the animal to move on as much as it breaks your heart.
She definately sounds like being an only cat would suit her better.
The only 2 other things i can think of:
* A behaviourist
* If shes an indoor/outdoor cat buying a large cat shed and run and giving her that as her own "house" with free access to the world.
The only hard thing about homing cats with this type of history is finding someone who will believe you that when she has a home with no other cats the problem is very likely to stop, afterall who willingly goes through what you have for a cat they have no emotional bond with. If you offered to take her back if the problem didnt stop that may help them to trust you.