But they specifically state that changing the past does NOT change the future that's already happened. It just creates an alternate branch. I don't know if they can entirely avoid creating branches... the very act of them BEING there should do that, whether they mess with anything or not. But they can try to prevent disastrous alternate branches that occur from the stones being unavailable.
So for instance, Banner wants to take the time stone from 2012. But if he does that, Strange won't have it to defeat Dormamu five years later. So they have to bring the time stone back to the exact point so that everything continues to happen as it's supposed to. They're not changing the past, they're changing their own future, while preventing that alternate devasatating branch from happening.
As far as Cap is concerned, he may return the Space Stone to 1970, but he does not stay there. Present Cap goes back to the 1950s (the cars we see at the end are from the 40s/50s) to live his life with Peggy. However, Past Cap is still actually in the ice at that point. So there would technically be two Steve Rogers'. Present Cap can live his life and grow old with Peggy, meanwhile Past Cap can still get defrosted in 2012, and go on to defend New York and team up with the Avengers if needed.
In fact, when we see Steve visit older Peggy in Winter Soldier, we find out she got married, had children, etc. We see the photos, but no photos of her husband. Perhaps her husband was other Steve Rogers' the whole time (she does show signs of dementia during their conversation).
Of course, that theory then starts to get a little weird when you consider Steve goes on to make out with Peggy's Grand Niece, if his future-self is actually Sharon's grand-uncle.
And you also have to sort out in which timeline Thanos no longer exists, because 2014 Thanos goes into the future and dies, which means he isn't there to reign terror from 2015-onward (no destruction of Xandar, no destruction of Knowhere, etc). So there's now a timeline where none of that stuff happened.
The timey wimey stuff gets pretty tangled, and often it's best just to wink and nod and roll with it.
