The writers of the film have come out and said that there's one timeline in the film.
And, curiously, the directors Joe and Anthony Russo take the view that Steve lived in an alternate timeline, but made a return trip to the original timeline to hand Sam his shield. This is the view supported by the film as Bruce declares that you cannot change the past to alter the present; travelling to the past makes it your present. If you leave the past and return to your point of departure, you return to the original timeline which has none of the alterations made during your trip.
This is supported entirely throughout the film: Gamora's past self travelling to the present does not undo her actions in previous films. Thanos travelling to the future does not undo the events of INFINITY WAR. Loki escaping custody does not undo the events of DARK WORLD or RAGNAROK. The only reason the Avengers need to return the Stones is to avoid creating destruction in a parallel timeline even if they'd never experience that themselves.
My guess is that the writers scripted a scene that declared that only removing the stones creates an alternate timeline and that time travellers cannot alter the past, but they can make supplementary additions. This view of time travel is present in DOCTOR WHO with "fixed points" in history where the Doctor must follow how history is recorded but can add details to pay off later.
I imagine that this scene was then cut by the Russos, resulting in two contradictory opinions between the writers and directors.
In the movie, when Steve and Bucky say goodbye, Bucky is clearly aware before the younger Steve time travels away that they are making their farewells. It indicates that the older Steve made an earlier visit to Bucky to explain the situation before his return to greet Sam. This is reinforced when Bucky directs Sam to speak with the older Steve and in fact seems to know where the older Steve will be before he even appears.
As for choosing Bucky or Sam as the new Captain America -- all I can say is that there's a certain value to a black man wielding the shield, but Bucky in the comics was extremely popular as the replacement Captain America. Maybe they could alternate every other week.
One thing I'd like to see -- I'd like a three hour movie, CAPTAIN AMERICA: STONE UNTURNED, where Steve returns the stone to be guarded by the Red Skull. In the first hour, Steve lands on the planet and finds the humble, repentant Skull. Refusing to hand the stone over to a man Steve knows as a Nazi mass murderer, Steve turns away, but when attempting to leave, the Skull attacks him, demonstrating Tesseract powers.
Steve is beaten half -to-death but is then saved -- by the Red Skull -- another one. The hostile Red Skull attacks them both and Steve and the Skull who saved him are trapped in a sealed cave.
In the second hour, we see the Skull and Steve with nothing to do but talk. The Skull apologizes for his misdeeds, but Steve won't hear it, saying the Skull isn't sorry. He just lost his power and whatever's outside the cave may be a time traveller or from another dimension. The Skull says he was made by deranged fascists and Steve was made by kindess and sacrifice that the Skull mistook for weakness. The Skull has changed. Steve replies that if the Skull could get his power back and return to Earth, he would instantly resume his campaign of terror. The Skull confesses that is true and asks Steve to share his exploits since WWII.
In the third hour, to pass the time, Steve describes some of his adventures and the Skull tells Steve that the Skull finds solace in knowing that the evil of the Nazis has, in a very small way, contributed to the good of Captain America. Steve replies those are easy words when trapped on a cave and also on a planet from which the Skull has no escape.
In response, the Skull leads Steve into the caverns of the cave which reveal numerous dimensional portals, some of which lead back to Earth. The Skull found them after the first several centuries of his imprisonment. He could have left at any time since then but chose not to, wishing to pay for his crimes with his isolation and fearing he would resume his old ways if he left.
At this point, the Tesseract powered Skull outside the cave finally catches up to the imprisoned Skull and Steve. Steve fights the Tesseract-powered Skull but eventually realizes: this isn't the Skull, it's a manifestation of his hatred for the Skull brought into being by the stone. So long as Steve does not forgive the Skull, he cannot give up the Stone.
Steve chooses to let his hate go. He forgives the Skull. The hostile Skull disappears. The stone leaves Steve and is returned to the Skull once more. The Skull shows Steve the portals once again and says that while the Skull will remain, Steve might make use of them now or in the future. The Skull wishes he could tell his mother good-bye, the Skull wishes he could visit each of his victims and apologize. But he can never allow himself to leave.
Steve looks at all the portals, seeing the 616 universe, the Ultimate Universe, the Spider-Verse, CAPTAIN AMERICA TV movies, the direct-to-video feature, the 90s Captain America who guest-starred on the SPIDER-MAN TV series, the HEROES REBORN Cap and others.
Steve also sees a portal leading back to Peggy Carter and one leading to the time machine in his original timeline. Steve says he doesn't know which one to choose. The Skull suggests that he choose both.
Three hours is too much? Oh. Well, maybe this could be one of those MARVEL ONE-SHOT short films. I guess a 15 minute length might make more sense.