Is Sephiroth actually Jenova?

[The Crystal]

Oh my my my. Some real clearing up needs to be done here.

Let's start from the beginning shall we, and lay down some facts.

Arrogant as always.

Will you never be tired of presenting your opinion and speculation as a fact, on top of your high horse?

Next, Jenova came here via space travel. However it skipped the expected space-ship arrival and came to the planet without anything else than itself.

Wrong. Sephiroth himself said he would use the planet as a vessel to travel the universe, like Jenova did a long time ago.
Jenova used planets as space-ships.

In reality, it was manipulating the knowledge of previous lives/memories immersed within the Lifestream.

No:

*Jenova's mimic ability*
Jenova has a mimic ability which allows it to read the memories and feelings of others, then adjust its appearance, speech and behaviour accordingly to immitate what it has seen. Jenova once used this ability to get close to the Ancients and infect them with its virus, which killed many of them.

This ability is not limited solely to Jenova itself, for those who have its
cells within them possess it as well, though in an incomplete form.
Immediately prior to the start of the game, when Cloud's mind was shattered, he ran into Tifa and seemed to immediately return to "normal" (-->P.13); this was because the mimic abilities of the Jenova cells inside Cloud read her mind, seeing her memories of him, which were then combined with his own ideal vision of himself, fashioning a new personality for him.

See? Nothing to do with "the knowledge of previous lives/memories immersed within the Lifestream".

It could have done this easily by spreading it's own cells throughout the Lifestream and then making it's own cells attack the host. If you've seen FF7: AC, then this would be known as Geostigma.

Wrong again:

(Accompanying screenshot caption 1)
According to Ifalna, the Ancients infected with the virus lost their sanity and turned into monsters.

Geostigma don't turn people into monsters.

And:

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.

Not the Lifestream, but "living organisms".

Jenova's virus was NOT Geostigma. It was something completely different

To do this, they first injected members of SOLDIER, an elite fighting group attached to Shinra with Jenova cells, not only to see what effect it would have, but to get the Promised Land sensing ability as well. However, the Jenova cells had little or no effect on the SOLDIERs. This is because the mind and body reacted to the foreign substance and attacked it, destroying the cells before the cells could do anything. In an attempt to get around the body’s defense, the experiment subjects were submerged in Mako. This brought down the body’s defense—for reasons I’ll explain later—and this brought into being the Sephiroth ‘clones’. However, these ‘clones’ did not posses the ability the Shinra were seeking, so an unborn infant was directly injected with Jenova cells, to work in the alien cells with human DNA--truly fusing the two species together. This is the creation of Sephiroth.

Bull. Sephiroth was the first one to be injected with J-cells, to find the Promised Land. After they found out the cells granted him super strength, speed, and skill, they decided to inject all SOLDIERs with them.

Now, I'll get back to the power of Jenova. These cells have an adverse affect on human cells, changing normal human cells into something more like Jenova cells. This grants humans some limited powers of Jenova, thus the reason for flying, walking on ceilings, and being unnaturally strong.

Wrong. No humans(injected with J-cells) can do that. Only Sephiroth(the real one) could fly, and only after he became Safer Sephiroth(and in the end of AC).

However, Cloud, in some fashion it, is never really explained, manages to get hold of the identity of his deceased friend Zack, and thus thinks he is Zack but with his own past.

Wrong again. It's explained how Cloud got hold of Zack's identity:

Immediately prior to the start of the game, when Cloud's mind was shattered, he ran into Tifa and seemed to immediately return to "normal" (-->P.13); this was because the mimic abilities of the Jenova cells inside Cloud read her
mind, seeing her memories of him, which were then combined with his own ideal vision of himself, fashioning a new personality for him.

Obs: "a new personality for him" = Zack's personality.

It was the J-cells.

and I believe I'll leave it at that.

Yes, please.

Do a favor to us: get down of your high horse, and stop presenting your assumptions as facts.