Thinking on it again and the Loki thing ends up being more confusing. If he did create a new reality that would mean the previous one would be without a stone and thus it would end up destroyed. This means one of three things happens. A new reality is created and that one is screwed even if Cap brings the stone back, it has no major effect on the events of that reality and things somehow continue like normal anyway, or it does change the timeline of that reality counter to what the rest of the movie was talking about.
Of course this also gets me thinking. The Ancient One says her reality can't survive if a stone goes missing. Makes enough sense but Thanos destroyed all six stones in the main reality so shouldn't that have been a pretty cataclysmic event? Shouldn't that reality be on the way to destruction anyway?
Also did Cap have that shield when he went back in time? I remember he had the hammer and the stones. If he didn't, that brings up a whole other batch questions in addition to the ones his jump already has.
My take is that Loki didn't use the stone to jump into another timeline, as it doesn't and never did have that power, he just used it to escape in that timeline. So it still has all the stones, but also just has Loki on the loose for his Disney+ show shenanigans.
As the stones combined keep the universe in balance, he had to return the stones they borrowed so the other timelines wouldn't suffer long-term problems, like without the time-stone, Dormammu would come and win. But for the hammer this doesn't really make sense, I guess they are returning it as the Dark Elves are coming to attack that day, but knowing what the outcome is for the Asguardians, even with the hammer, just returning it to that reality, not helping them to change their outcome seems like a shitty gesture.
The shield sort of implies that the time-travelling Steve rodgers took on the role of Cap even in the other timeline, which begs the question of what happened to their timelines steve rodgers, did they just leave him in the ice or something? maybe they just made two of them. I dunno.
Thankfully this was the last film in the phase, as the moment you introduce the ability to take things from one reality into another you just open a massive can of worms. It doesn't even make sense how Thanos got back in time. They had nebula's wrist thingy and some pym particals, and used those to shrink down an entire army and send it across in one go? yet the avengers were using one per hero despite them going ot the same location and having a limited supply.
But it gets worse, as why the fuck was Thanos even stopping them in the first place? think about it - that Thanos was from another timeline, even if the avengers borrow one of the stones from his (the space stone ) to undo the click in their own timeline, 1.) they are going to return it, and b.) so fucking what, the thanos is dead in that timeline anyway, why send your entire army to another timeline rather than keeping the time travel device from the future and using it in your own timeline.