Mind's Eye true would be acceptable at C rank. It would entail that he would be able to more easily dodge but not counter as easily as Emiya did as Cu.

I'll be willing to let presence concealment stay at A. When combat initiates it would probably drop to C, or D the wiki doesn't give any hard facts but PC is not suppose to cover for servants who engage in combat it is meant to give cover to those who are stalking.

If you want to switch to Okita that is fine. I was planning on having her as the black NPC assassin irregardless.