Kishuna being Renault's friend is a big stretch. too big. In fact, impossible.
Nergal is skilled at making morphs look like people, while they do not have the original consciousness, it still looks like them. Kishuna looks nothing like any human being. unless you've tried running up next to kishuna with renault to see what happens, and there happens to be some hidden dialogue to prove your statement, there is nothing to suggest him being the reincarnation of Renault's friend.
What gets me is, if Nergal had no need for kishuna, why were there morphs still protecting him? (Yes, I know, its a game and they need some kind of conflict to make the levels worthwhile. aside from that...) Can anyone speculate to why that would happen?