Depending on how much they expand it I would put the end point for part 2 at either Nibelheim or, as you say, the end of the crater. I don't think they'll leave it at Aerith. I don't think they'll end that part on such a depressing note; the crater is a much better cliffhanger. Then you can pick up part 3 starting with Tifa and Barret in Junon, which would be perfect.