It may be pretty apparent that these are actors and that the event was staged by the team at Comic-Con, but the proposal itself has been confirmed to be real and the lovely soon-to-be Mrs. America was genuinely surprised. Looks like the first avenger may be spending some time without his shield in the near future.
The First Avenger: Captain America, actually he was an Invader before he became an avnger and the first avengers were iron man, hulk, thor, ant-man and wasp. Cap was initiated in the 4th avengers comic and became the 5th avenger with honorary founding member status.