As far as I understand there is no concept of multiple timelines in Avengers: Endgame. So when Tony and Cap are trying to bring back the Infinity Stones with the quantum tunnel, Tony lost it and Loki opens a portal and disperses. Doesn't that make the past more complicated (altered plot opposed to what was supposed to happen [Thor taking him back to Asgard and so on...])? Even if Cap takes back the Space Stone in the past, at which exact point in the timeline should Cap take the Space Stone to make the mission successful?
after Loki is gone. That would make 2 Space Stones in the past (not possible logically), one with Loki and one with SHEILD agents.
before Tesseract was handed over to SHIELD agents. Cap would have to face 3rd of himself, one from the past, one from the future who is bringing the Tesseract back to the future to destroy the stone.
It makes no sense as to how the Space Stone is returned.