This is a plot hole as confirmed by one of the writers of the Endgame. Russo brothers knowingly kept the scene to cheer the fans.
From the link:
Avengers: Endgame co-writer Christopher Markus has shed some light on a plot hole from the film regarding Captain America using Thor’s hammer, Mjolnir.
While Cap finally lifting up the iconic hammer was a ...
A few of things really:
Captain America was injected with the Super Soldier serum
In Captain America: The First Avenger we see Steve's origin story and how he became Captain America. In it we see Steve is injected with the Super Soldier serum which enhances the man and his morals. It also makes him taller, stronger, more muscle, better reflexes etc. Steve ...
He's Captain America after all.
Remember, last time Captain America was trained as a soldier, and the fact he's a super soldier makes it more possible that he can still be alive. Yes, though Thanos is probably more powerful than him, it looks he doesn't do much damage to kill him.
This is made pretty clear in Spider-Man: Far From Home, the event when people came back from Smart Hulk's snap was called The Blip.
Betty Brant: Historic. Over five years ago, half of all life in the universe, including our own Midtown High was wiped from existence. But then eight months ago, a band of brave heroes brought us back. They called it the Blip....
Sharon is not Peggy’s neice in the MCU; in the comics yes, but in the MCU no. Peggy only had one brother, Michael, who never had kids. So the whole Steve kissed Sharon who he is now related to is pointless. Sharon is not a Carter in the MCU.
The clue lies in the way how the Avengers found out where Thanos was in Avenger: Endgame.
Thanos used all the six stones twice in Avengers: Infinity War, second time during the snap, and the first time minutes before the snap while he was trying to block Stormbreaker.
RHODEY: That's cute, Thanos has a retirement plan.
STEVE: So where ...
He used the Infinity Gauntlet with the Infinity Stones in it to destroy the Stones. Not only is this the way he uses to use all the Stones at once but we have physical evidence that this was the case. When the Avengers go to Titan II in hunt for Thanos after they detect the Stones are used again we see the toll it has taken on Thanos. We see now, his left ...
I think you've fundamentally misunderstood the argument between the Ancient One and Bruce Banner.
When the Ancient one shows the effect of removing a stone from the timeline, there are 2 separate branches of time created - Main Timeline continues as normal once the Avengers return to it, and the Alternate Timeline starts off from when the stone was removed.