Is there a way to replay missions? I missed the opportunity to kill Hitler and also want to show a friend that amazing scene... Unless I am missing something it appears the only way is to restart the campaign and replay all the missions??? That seems like an odd choice.

You can't replay the actual story missions to my knowledge. You can however revisit certain levels using the enigma machine to find collectibles and uber-commander cards.

You'd need to load a save to or before the story mission to see it again.