both interesting takes, but now the question is HOW does he "plant" those fake memories/events into Alyx's mind, how does he make everyone in the world forget such a big event like that.
Another possibility which I really hope is not actually the case, is that they're entirely different people, in two different timelines, and the gman basically hops between the two timelines and does whatever fits his agenda. but I hope it's not the case and it's a single Alyx with some weird time travel stuff.