Sorry BAD ENGILSH, I`m from Spain:
Jenova takes Sefirot´s body form to manipulate Cloud, (In the Present)
In the scene of the Mako Reactor in the Past, Sefirot cuts the Head of Jenova *And take it with him, to the lifestream* (you can see in LAST ORDER OVA)
THIS> Explain the Sefirot and Jenova presence at the North Cave.
THIS> Explain that the Jenova of the Shin-ra building doesn´t have head.
I can say too, that the one who killed president Shin-Ra is Jenova, when travelling across sea, when you see Sefirot he fly away and Jenova appears. When Aeris dies he fly away and Jenova appears. And you fight jenova´s parts one by one, except the head. (That is on the North Cave, with the true Sefirot)
NOTE: I have to thanks SquareSoft for the traduction of shit they made to the Spanish for this game, a shit, a real fucking shit... XD