I dont see why it couldnt be sent back and forth, or why things couldnt be changed. If they suddenly took Aeris out of the end when they were deep into making the game, it wouldnt do too much, depending on where they were in the story. I'm sure every script/plot is tweaked around almost constantly. If it's set in stone, then how could corrections and such be made?
There's only one thing we know for sure... VII was under serious time constraints. It had to effect something.