I really liked how the trailers basically spoil absolutely nothing of importance, and about 10 minutes you have no idea how the movie will go, and I don't think anyone predicted this. I guess that is subverting expectations in the good way.
Up to Old Steve Rogers on the bench, it made perfect sense to me.
You can't change the past. It's set. When you travel in the past,you are really traveling to a parallel universe.
You can do there whatever you want, including stealing stuff, and use it for your own purposes, it has no effect on the timeline you come from. Of course, you might screw over that universe if you steal something important, like the Infinity Stones, but you can also help them a great deal if you remove something dangerous, like, say a guy hell-bent on wiping out half the universe. So basically, their actions saved actually two universes (or the population worth one complete universe), because they undid the snap in their own timeline, and removed Thanos from the other.
But I can't quite reconcile how that works for Old Steve Rogers. I mean it's possible there has always been a Steve Rogers that came back at some point to reunite with Agent Carter (clearly after the events of the series), and that's our old Steve Rogers now. But that suggests that travel into the past doesn't always end up with an alternate timeline. But how would the time travel mechanism "know" which option to pick?