don't forget your Jenova party members also change between the first and third phases (where 2nd phase = sequence of 2-on-1- fights). In hard mode I had everyone at 100% compatibility so I can't provide any insight on this but my previous theory on whichever party member isn't in the main Jenova rounds will be in both Bahamut & Wings fights was proven true and, just like I wanted, it was Tifa. I had also gone back and replayed chapter 12 with Tifa as my date partner just in case that made a difference, might have had an impact but might've just been random.
but yeah I think I had posted earlier, and I had continued tracking the final battle teams of any streamers I was coming across, but I deleted all my tracking once I got my platinum which was dumb of me but too late now. I do know people with the same chapter 12 date didn't always have the same final battle formations, just couldn't find a recurring theme.... Yuffie being a constant for the wings fight might have been a thing too, I think
Might spin up my first game clear save file tonight just to check, since I had the.... ugh... Red XIII date in that one, so top compatibility at game's end. Will see how my compatibility stats for the final battles there match up w/ the character ratings.