Yeah that seemed the most likely to me. I'd guess it's a flashback for Hawkeye, to what he was doing at the time of the fingersnap.
Edited to add: Although I could see training someone that lost their parents because of the fingersnap, being something that helps him cope. So a possible Kate Bishop that way but one that would be undone by going back in time to stop it all from happening? mhm the fun of random guesses! It'll be interesting to see who she actually is and how it is really done in the movie.
Well, we know that Tony and Nebula make it back to Earth and that the entire team is back together. I find it a little telling that Steve is the one leading the crew to wherever they're going. They also don't show Rocket in that procession although he was shown in the Big Game spot. There's someone behind Natasha, I think it's Thor. Don't see Carol there, but as with the first trailer we don't see Ant-Man until the last part.
I do like when Carol does show up. Thor casually summoning one of the most powerful weapons in the galaxy and her reaction being quite blasé is a nice touch.
On the whole it wouldn't surprise me if everything we've seen is only in the first hout/two acts of the movie. The action stuff we see with Scott could be his escape journey to the Avengers compound, the Nebula scene looks like it could be on Titan and Clint could be where he meets up with Natasha.