Like I said, that's not something new on the show, Episode 9 already introduced parallel universes. About ep 12., I don't think there's much to understand besides it being used as a tool for Ai to face herself and deal with her issues. We can debate how good or bad decision was (I ended up liking it) but I thought it was pretty clear what the point was.
I didn't think Kotobuki, someone who appears for half of an episode, mentioning parallel universes was much of an introduction. It was still a surprise to see two Ai's in the same universe triggered by...smashing an egg...because it hadn't happened until that point, and there really wasn't an indication that it could until it happened.
To me, it was a messy and rushed way to conclude Ai's arc.