Zack is not one of the clone thingies. They are inside of the metal containers in the Nibelheim reactor. Him and Sephiroth are standing outside those containers, looking in through the little glass window. Zack cannot be two people at once.

It does not mention anywhere in the game that Jenova can grow back lost heads.

And, regarding:
The coffin lid is wood. Not metal. IF it were metal, it would be shinier, unless it were made of carbon, which is just ridiculous.
Or unless it were made of cast iron, like a lot of coffins. Or copper, which oxidizes into some green stuff. Or unless the coffin had been lying there for a few years without dusting or polishing. Metal is not always shiny. On the other hand, pure carbon is definitely shiny.

You can't fill up someone's limit gauge by making him angry. Otherwise Barret could do limit breaks all the time...

Off topic: What part of
I am not trying to get personal, Nick.
didn't I make clear? Don't be paranoid. If we would really hate you, would we communicate with you?