“After another read through the manual, it seems still that FCP doesn’t let me take a Synced up timeline and cut with it. The multiclip sequence feature only works if the rushes timecodes all match.”
I’m not sure what this means, but the second sentence is obviously incorrect. You can match in points and make them a multiclip regardless of timecode.
If the cameras are drifting that’s a different issue and has nothing to do with multiclip. Is it a non-drop/drop error. Two different counting systems will cause slow drift.
I didn’t seen anything on how the timecode was set, but you might be able to sync up, modify the timecodes and then the multiclip sequence function.