Err I think given there's enough of her to make three huge monsters then dropping pieces is just a slowing tactic either way. Jenova WANTED Cloud at the reunion.
Why would Sephiroth assume his own form. Jenova at least throws people off her scent, Sephiroth would just be letting people know he's alive even though the thing you follow uses stealthy techniques at many times during the course of the game. There are other reasons for Jenova to look like Seph too.
Sephiroth never begins to obtain the god-like powers, meteor never hits.
Sephiroth killing the clones makes no sense at all, why Jenova would do this has already been suggested, and to be honest is the only logical explanation I can think of.
I think the black materia did more than just summon meteor, maybe not directly, but Seph doesn't do anything for years before coming into contact with it.
Jenova didn't do anything till then because she was trapped with no way out. Not because she was weak, it took a hell of a lot to get her like that, but when the oportunity came, she got out and all hell broke loose.
I don't think the head is relevant. Given Jenova can communicate to all of her body parts nomatter how far away they are I just take her to be a single entity in different places.
