Okay first off I want to aplaud Nick for sticking up for himself on his own for so long. I don't know if he reads this still but I'd just like to say that as far as all the arguements have gone so far, he WAS in the right!!!
Maybe not about Zack, I'll go into that in a second, but he was saying that it was POSSIBLE, everyone who said it was impossible or almost impossible or nearly impossible is WRONG as far as arguements have been so far, their are enough valid arguements for it to be at least plausible enough for fanfic.
Anyway now onto the topic.

As far as the scientific arguements have gone there are three arguements that just don't hold so in final fantasy should be discounted.
First, Vincent wouldn't have been stopped by the door.
YES HE WOULD!!! This is final fantasy! A team who has taken on Shinra killed jenova's armoured killing machines and travelled all over the world CANNOT open the door without a key!!! So Vincent could very well have been called.

Second, Zack couldn't have survived the gunfire. YES HE COULD!!! Again this is final fantasy, people only die if they are meant to, nomatter what happens to them. Cloud, Squall, Vincent, Dyne, Ward, Kiros, Laguna, etc. all have survived surefire deaths. Zack was not certainly shot in the head, and even then he could still have survived.

Third Jenova can be killed easily, says who? Even if it doesn't revive itself, thats because you destroy it! You can't regenerate yourself if you have a starting point. If it's not destroyed, who says it doesn't revive. Maybe you don't see it, or maybe you do. Doesn't Synthesis suggest that the previous forms have joined, it doesn't really seem the logical step after Birth, Life, Death does it?

But thats just arguing against theories trying to disprove the possibility, I'm not saying the man in the pipe is Zack, or that Vincent was called, although I have to say Jenova cells do seem the most likely source of his powers and survival. It is plausible enough for a fanfic, and I believe that is how it was intended (open ended), or maybe as has been suggested, just as an early glimpse of the clones.

I'm sorry to all the people who think that this topic should be left, I am not argueing that I am right, merely that other people are not wrong.