Mako Eyes said:
1.) Geostigma is gone now that Sephiroth is dead.
Sephiroth’s will is no longer influencing the cells, but the cells themselves are still there.
2.) Jenova cells can no longer survive or exist within the Lifestream because of Aerith's holy water.
This is a big assumption you’re making even though we haven’t really been given any kind of confirmation on what Aerith’s rain actually does. For now it doesn’t seem to be any different than Great Gospel, which grants subjects full recovery from all ailments. The skill has never been shown to be capable of actually killing J-cells, and I don’t see anything beyond subjective analysis saying that the rain did in fact kill J-cells at the end of ACC and supposedly went on to spread throughout the Lifestream.
Based on what it did in the original game and what we see in ACC, the rain appears to have just healed the damage of Geostigma for people beneath it. Nothing more. We are never shown or told that the Lifestream has suddenly adopted the traits of Great Gospel permanently. Aerith did not suddenly turn the whole of the Lifestream into a pool of miracles.
3.) Sephiroth's will is gone. His spirit was destroyed, as was his physical form made of Jenova cells.
Sephiroth’s status post-AC is not what the discussion is about. The discussion is about the possibility of Jenova’s return, not Sephiroth’s.
4.) Jenova's main body and head are gone, leaving behind only a few scattered cells. Her mind does not exist within those cells, meaning it can't miraculously return to life and become whole again. No Reunion instinct is at work instructing the cells.
You still haven’t provided anything indisputable pertaining to impossibility of Jenova’s return thus far. No description of Jenova has ever said that it needs its head or its main body to perform Reunion. The individual cells themselves call out to one another. It’s an instinct within the very cells. They wouldn’t need a sentience commanding them to perform it. They would simply mass together until all pieces were reunited.
And I don’t know how far you wanna read into this “sentience” thing. Jenova was a creature of pure instinct. Any intelligence it had was dedicated solely to mimicking and manipulating its victims to further its drive to cause destruction. The Reunion instinct is something basic that is present in each and every cell, independent of whatever higher thought processes Jenova might’ve had, and it’s this quality that warrants its description as being practically immortal and abnormally resilient, and what pretty much condemns the planet to having to deal it again sooner or later.
Final Fantasy VII Series Terms and Definitions said:
Reunion
[FONT="]「[/FONT] FFVII – AC [FONT="]」[/FONT]
One of the properties Jenova possesses is that separated pieces of flesh, called cells, will regenerate back to their original form - working toward the phenomenon of reunification. This is shown by instance(s) when both Jenova, who is practically close to immortal, and the son whom should be called Sephiroth, are resurrected sooner or later even after being destroyed.
Final Fantasy VII Ultimania Omega said:
Vicious, extremely aggressive and cunning. Having no thoughts of creating anything following bringing destruction, it only follows its instincts to incite ruin, utilizing its mimic ability (-->P.211) to infect other living organisms with its virus. In addition, due to its abnormal resilience, even if dismembered, its cells will call out to one another, seeking to reunite into their original form (-->Reunion/P.211).
Final Fantasy VII Ultimania Omega said:
**Jenova Relation 2: The Reunion* (005.1B)Due to its amazing regeneration capabilities, Jenova's divided cells reunite to become one again.
I figure there’s enough info to justify the idea that Jenova does have a chance at making a return, with or without a head or body. Though I don’t see enough information that suggests Aerith’s holy water can destroy Jenova cells. Nothing says as much, and the only part that so far as only implies it can be interpreted as being nothing more than a recover from the damage and pain rather than a destruction of the actual cells.