Gamora was a real win in this movie, she's come a long way as a character. Interesting how much she has in common with Loki as well. I like your idea of how she's going to become the de facto leader -- she proved it was her role to take in this film.

However I'm not sure she did know the specifics of the soul stone's price, just a suspicion based on Thanos' response, but he might have killed Gamora for various reasons.

Yes, I think that Pepper was always meant to be younger than Tony, and that both characters were meant to be younger than the actors actually were.

I don't know about May (or Ned or MJ for that matter). I'd like to think that she was, rather than realize Peter was gone.