Summarizing the whole thing… yeah, not happening. Your specific questions, I can give a short version of:

Seph, Cloud and Zack were all subjects of genetic experiments using Jenova, which was an alien creature, not really compatible with creatures from the planet. Seph was so changed that he was influenced into being Jenova’s servant, and Cloud was affected to lesser degree. As for Advent Children, Sephiroth is dead, but his life force got into the Lifestream, same as everything else that dies on the planet, and was called out by his clone followers.

Long version… go to Gamefaqs or something.