If Jenova's head does become Sephiroth, it might explain (SPOILER)what happens to it when Kadaj gets upset over its "disappearance", in the church--you know, he clutches the box thingy and wails "Mother!". But I'm not sure if that makes sense, because if it became Sephiroth (or rather, took on the form of Sephiroth), wouldn't the SHM know about it?