No the sephy at the start of the game is a part of Jenova disguised as him, making it's way to the reunion. The real Sephiroth IS the child of Hojo and Lucrecia with Jenova cells injected into him.

And he didn't DIE in the lifestream. He was just frozen. If he was dead he couldn't summon Meteor and you wouldn't fight him at the end.

As far as we were concerned Sephy was dead. Now Advent Children tells us that He'll never be dead. Do you not see that all Square are doing is giving themselves more to work with in the story so they can milk it even more? They actually said that the compilation will go on for about another 10 years.