The way I see it there are no multiple timelines(strictly in Endgame's context). Prime Universe IW snap was undone & that's it. Anything extra that happened in Prime universe & other "temporarily created timelines" were also undone by the end of the movie. Thanos did succeed but also died twice in prime universe(1st after IW snap & 2nd after coming from past of prime universe). The entire premise of returning infinity stones to their original places in past after undoing the snap was to prevent occurring of alternative timelines.
Anything opposite to above is simply plot hole if marvel does decide to bring back loki/gamora/iron man/black widow/vision in future movies of this phase of marvel movies.
The writers and directors do not seem to agree if there were multiple timelines or not.
We were just shown conceptually how returning the stones to the time that they were taken from, but not practically. He has to return the reality stone and put it back into Foster just after Rocket puts the Asgardian guards on alert. Returning the time stone to the Ancient One should not have been much of a problem, but when it comes to the power stone, it has to be done before Star Lord gets knocked out? Or Star Lord just does not remember getting knocked out? And really, what about the soul stone, he just shows up on Vormir and tells Red Skull, here take back the soul stone, we do not really want it anymore. Does he get a soul in exchange for the soul? After doing all this he jumps into the past and gets married to Peggy Carter, although she already had another husband that Cap rescued. (Although many MCU chars have appeared in TV Series, think Jarvis is the only character from a Marvel TV series to make it to the films). As Peggy Carter's husband was never mentioned, it could actually be Cap, although it seemed like it was another soldier that Cap rescued. It is just cryptic enough to be plausible.
In any case, the stones cannot be returned at the exact time, and there are slight changes in the universe, at least that would lead to a butterfly effect as time passes.
So where did Cap go to return Tessaract? 1970 or 2012?
Assuming he went to 1970 to return Tessaract and reunite with Peggy, they didn't close the timeline branch they created in 2012 where Loki took it and escaped.
Yeah, he cannot "fix" both the timelines.
- Cap jumps back? If I have to assume, I would assume it as that he stayed with Peggy and got old. But how that timeline crossed prime universe is kinda loophole. If he stayed with Peggy, he didn't fight 2012 war (or he did fight as an old man), or who saved Bucky in Winter Soldier?
- Gamora 2014 is alive in prime universe. So in that reality Gamora doesn't exist unless she goes back there.
Yes, exactly, plus Peggy has to pretend that she is not married to Cap when she meets him at her death bed.
Also, this is hilarious - all the problems with undoing the snap.