So far as I understand it; according to quantum physics the entire universe "splits" every time any particle interacts with any other particle, anywhere and at any time. So effectively infinite branching, everywhere and at every time....unless there's some weird void in the universe with zero particle interactions.
Of course, that's not how it works since "splits" implies a linear path that can deviate rather than what's probably really going one which is everything existing everywhere and at every time and our perception of causality is just a chain of infinite collapsed wave functions along a 4 dimensional light-cone.
Time travellers and indecisive restaurant patrons do not have the magical ability to create new timelines anymore than a boat creates the currents in the ocean. It's all always there, it's just a matter of navigating the infinite, which is what Stark's gizmo did: provide a means of returning back to the *exact* point of origin down to the planck unit.
Infinite time zig-zags.
They didn't return to the "exact" point of origin.
Spatially yes, but it was hours later.
What that means, who the fuck knows, but at the end they said that Cap would only be gone for 10 seconds, and Steve had the will to fight that. Meanwhile in the middle of the movie, all the heroes arrived home at the same time even though their adventures were almost probably different lengths.
Thanos arrived later. So he overcame the pull of the anchor in the platform air traffic controlling them home.