I think an animal behaviorist would be of help.
(not an animal psychic )

Try putting a bell on Butter. That will give Mimi warning as he approaches.
Also, try confusing thier scents by putting a drop or two of real vanilla at the base of thier tails. (on the top side, not by the butt)
It sounds like you have a potential 'pariah' cat situation.

I would have Mimi to the vet as well. I think that in the animal kingdom, they can smell weakness, through disease or injury.



Sorry, I can't be of more help.

That is a very scary scenario. After years of cohabitation.