Read this before posting for all story and DVD/UMD questions (Spoilers aplenty)
cory droid: Another possible interpretation is that by “mimetic legacy” the inhabitants of the FFVII world (Gaia?) mean to refer to the mimetic properties of her biological material: how they project/morph into mimes of the thoughts of those they come into contact with. Sephiroth’s genes and Jenova’s cells both can do this.
...On one hand, we have remnants of Sephiroth’s thoughts, on the other we have cells which can read thoughts and actualize them. If from the combination of these two, we get a physical manifestation of a deceased(and remembered) being, then it’s not too far out to suggest that the product of these two is the result of said mimic ability.
None of this addresses one of the questions I originally posed: we know that Sephiroth's will was actively goading the SHM to find Jenova's remains so that he could come back, and it was also its resonance in the Jenova cells that prompted people's bodies to begin seeing Jenova's cells as an invader that needed to be expelled. Which was all according to Sephiroth's plan to contaminate the Lifestrean and control the planet.
Your suggestion -- if I understood it correctly -- calls for disregarding the notion that the Sephiroth Cloud fights at the end of AC was aware on some level all this time and planning Geostigma. It even disregards the notion that this is actually him. That this mimicked Sephiroth would just randomly appear and know all the things he knows while the real him is unaccounted for undermines the basic plot elements of the story. Unless you're only trying to suggest that he was able to give the cells his form because of their shapeshifting attributes, in which case, all of us expected nothing else, I'm sure.
In any event, Kadaj is part of Sephiroth's spirit, his living will. He is a physical manifestation of this, and when his energy bonded to Jenova's cells, Sephiroth returned. This is more than just the cells randomly mimicking things. This is Sephiroth's spirit entering a physical vessel that allows him to live again.
cory droid: Anyways, the line reads “5 years ago, Sephiroth fell into the Lifestream...and where he would begin the Reunion...by focusing through Jenova's cells.” If you parse that, you get two subjects, one who does the action (Sephiroth) and that which the action is taken upon, or through(her cells.) Does this read the same in Japanese?
Yes, it does, but I'm not seeing the relevance of the matter. Jenova's cells are largely a plot element. They're called "Jenova's cells." That's where they came from, what they're called. Randomly renaming them wouldn't make sense. They continue to be referred to as "Jenova's cells" even into AC, where they're given a correlation with Sephiroth's "element."
cory droid: No, they don’t. The sense of wonder and mysticism invites itself into the work of art. You can’t blur the line between fantasy and reality, just so that you can arbitrarily choose which explanations of things are true in order to support your ideas.
You can't assume that everything has been changed from the status quo of the real world until we're given reason to either, or it's impossible to view a fantasy story. If you assume that people cannot fly, that they cannot breathe underwater, that they have the same tolerances to heat and cold that we do, then things are a lot more logical and run a lot more smoothly.
==FFVII Ultimania Omega translations==