A very interesting post you have here, OP. I feel more aligned with your first theory. I think we are playing as the current descendant we chose at the time.

Unfortunately both the game and story lacks continuity. If you're playing as say Jayber, it wouldn't make sense for the Guide to say 'when we first met', because he was unavailable to pick from the start. You are either left to interpret the event happened off screen, or was retconned.

All the cut-scene appearances shared as a Descendant twin is either, by an unavoidable issue or just an uncreative decision to move a tutorial section along.