Boy, what a tough choice. I can't help thinking in terms of my own cats. One I would take with me, the other two I'd leave with my willing friends. If they were the same age as Kit Kat, mine are all older.
I guess I'm no help but you seem to have thought this out very well. I sort of wish Kit Kat had stayed in her first home. It takes a while to get used to a new dog. The cat I'd take with me pretty well hid from our puppy for the first year but now they friends.
I guess that's not an opton? Going back to your brother? I do sort of worry about the claws versus no claws. It's why we did not keep a lovely stray manx, he was declawed and had no defense against my wickedly territorial cat, the one I'd keep with me.
Good luck in your decison, I know you will make the best one you can.