Questions about ff7 (spoilers)

[Sephiroth]

Thanks a lot skyblade!!
I never thought that he kept cloud alive so that he could get the black materia.

1. What is the jenova reunion theory?
2. How does sephiroth have control over jenova?It should have been the other way
cuz SEPHIROTH was injected with jenova cells.
3. What are the voices inside cloud's head?
4. Where was the reunion supposed to be held?Why didn't the other clones make it?

1. When Sephiroth took JENOVA's head Hojo ordered to bring her body to the headquarters. He was studying JENOVA and the reunion. He thought the body would be the core of the reunion and would activate it and that JENOVA would return to the headless body but he was proven wrong when Sephiroth activated the reunion and the clones, et cetera wandered to the Northern Cave.
2. Actually Sephiroth was never injected with JENOVA cells. The fetus explanation is just an easier explanation which was also mentioned by Hojo in Final Fantasy VII and later in the Compilation. The complete and more "complex" explanation is: Sephiroth himself never got the cells injected, there was no fetus of him at that time. Lucrecia got them before she was pregnant and they immediately fused with her and Hojo's cells, so right during the fertilization. That is the reason why there is a "JENOVA inside her" like she says. She has the cells inside her body so the cells where not "injected through Lucrecia's body into Sephiroth's body" but the cells were injected inside Lucrecia's body and the fusion happened immediately when the other cells fused; he was an experiment all the time and Hojo did not care about his child being an experiment.
Since Sephiroth was a complete success he is able to use all of JENOVA's abilities. Sephiroth is stated to have the most powerful will which is required to gather even more information and energy without the own mind being broken, like it happened with Cloud when he fell into the lifestream. This willpower and being a successful "heir of JENOVA" allows him to fully control the cells.
3. There are two voices which try to tell Cloud what is true. One is Cloud's subconsciousness, the part of Cloud who knows that not everything he remembers has happened the way he remembers and also warns him and the other one is Sephiroth trying to use his weakened mind for his own purpose.
4. At the Northern Crater where Sephiroth called out for it and where JENOVA's head was. The clones actually all "made" it. They made it as far as it was meant to be by Sephiroth. It is just that not all of them were shown. The reunion called the Sephiroth clones and they came to the Northern Crater. He wanted them to be there so he could kill them and reunite JENOVA, the only being similiar to him while he was able to use the reunion for his own purpose like wandering around with the Sephiroth body replicated of JENOVA's headless body with his shapeshifting ability. Other former SOLDIERs also got JENOVA cells but were no part of the "clone Sephiroth try" were affected by the reunion like the seller in Junon but just were not "manipulated enough by Sephiroth". Only the clones so most of JENOVA's former cells except some like those Sephiroth was made of, Cloud's, the Junon seller, Lucrecia's, Hojo's and some others were all used to reunite JENOVA and JENOVA Synthesis was the result. The almost complete form of the true form JENOVA has.