If I went into space for a long time. And Then returned to Earth, would I be an Alien?
Jenova was never said to be originally from Gaia, if that's what you're implying.
Yes she can change shape threw diffrent means. But her true form is female.
I'm pretty sure this isn't stated anywhere.
Replace "him" with "her". Now swap Jenova and Sephiroth.
The Ultimania guide states that Sephiroth is powerful enough to resist Jenova's control and overcome her.
It also says that Cloud was the 1st Soldier who whitnessed what Sephiroth did. (It was Zack). People believe things and are mistaken. That's Classic Final Fantasy.
Yes, and that is explained later in the game. And I don't understand the second part.

Just a bit of trivia. More may come in later threads or this one depends on the situation.
In the future, you may want to make your trivia more accurate.

Inportant Notice: Whatever Is said in this thread or anyothers by me, does not inclued Advent Children, nor Derge of the...whatever. Because they may not be connected as there were different Square members. So don't use theoes 2 "Chapters" as argument.
The Compilation of Final Fantasy VII and the Ultimania guide are canon, so I don't understand this either.

Really, they've made a book to help explain the games, so I don't understand why these threads appear from time to time. If you want a translation of the UOG, go to Gamefaqs. This shouldn't be so difficult. It's just a game.