There are some things to consider. Basically all my beliefs and speculation on FFVII is based on FFVII the original game in USA only. I've played Dirge and watched AC, but...idk, I don't really WANT to count any of that information as being valid. As far as a Jenova Effect - the Jenova we see in the game is the semi-dormant Sephiroth influenced Jenova with its head cut off - so I don't think it was actually mind-fucking Tifa or Cloud, but it was a convenient way for Seph to play off of Tifa's and Cloud's damaged memories. It would be interesting to see what the Jenova that migrated to Gaia was like when it arrived. I've always held the position that Jenova taking the image of friends and family applies to the Ancient Jenova (Cetra folklore). Seph took Jenova's head and got tossed into the lifestream with it, and thats why we get to see Jenova Birth/Life/Death/Synthesis, and The Reunion is definetly real, but other than that, I think the Jenova of the FFVII Present was just a dormant mass of powerful alien cells - The crystalized Sephiroth in Northern Cave was the being in charge (as opposed to Jenova or the Seph 'image').
To recap - Sephiroth begins life as a Jenova-Cell injected fetus. He may or may not have been given a Mako Shower at some point while in SOLDIER (though he likely did not need one, or it had little effect). He tweaks out after realizing he was an experiment. He cuts of Jenova's Head and Cloud then tosses him and the Head into the Mako pit under Nibel Reactor (presumably connected to the Lifestream). Eventually Seph arrives at Northern Cave, chills there becoming a Crystalized Mako/Jenova Cell/SOLDIER hybrid. He sends his ‘image’ out to unlock Jenova’s body, Reunion begins, clones go north, Cloud goes north (though I think he was truly just chasing Sephiroth ‘image’ and not being summoned for The Reunion [but Seph says, “Are you participating in the Reunion…I don’t think you’re worthy”]), Seph gets Black Materia. Crystallized Sephiroth is the real player behind the scenes, we only see him that once…but that is the being that is in control of all this.
Once Jenova is sealed away by the Cetra, that’s the end of Jenova-proper. We fight Birth/Life/Death which could be ‘images’ or mindless Jenova Cell masses, or perhaps Jenova itself acting independently, or is being controlled by Crystalline Sephiroth. After the Reunion, we can assume Jenova has reclaimed all it’s cells, save for the ones in Cloud, and Red (if he has some), and perhaps others. I don’t know what I was saying anymore…basically the Crystal Sephiroth is the man in charge, the Jenova we see probably isn’t the Jenova on the Ancients’ Past, and as such, any powers that Past-Jenova had, probably don’t exist in the FFVII present, but Crystal Seph may have these powers too.
