No, it isn't, and no, they haven't. The movie clearly tells us that you cannot live in your own past. Period. No exceptions. And being able to see 14 million different outcomes for the future sure as hell makes you omniscient. 1) is a blatant lie, man. We are told specifically that you cannot go to your past and take action there. They can go to the past, which is an action by itself, and thus create a new future. Which Cap going back and meeting Peggy is. And 2) Is utter bullshit. Why would you be able to alter your past, but only in certain instances? What is the difference?