No. Once you sacrifice someone you love in Vormir, you get the Soul Stone. If you sacrifice another person you love you don't get anything else, because there is only one Soul Stone per universe.

Hulk's commentary says that once you go back in time you don't change your past or your future, you create a different timeline, whose events can go different depending on what you do there. When the Avengers time-travelled to New York in 2012 they clearly created events that didn't happened in the past, like Loki escaping (an entire plot point to the Loki series, as well, which did not happen in the main timeline, clearly, and one show that explains that changing time is an inherent characteristic of the MCU's multiverse). Also in 2014, when young Nebula discovers her older 2023 version, and Thanos and Gamora from 2014 time travel to 2023, they do not return. You can't have Thanos leave 2014 to die in 2023 and expect the events of the rest of the MCU to continue the same with that. So yes, the Avengers do create alternate new events that do not converge with their own timeline.

In regards to the Soul Stone, the time-travellers can indeed time travel to any point in time when the stone was on Vormir and obtain it, but they can't obtain the stone again if they are in the same timeline (unless for some reason they decided to put the stone back in).

To put it simply; what you do to the present, can't effect the past. And since wherever you are in the moment is the present. So if you're in 2023, and go back to 2012, you're experiencing 2012 in the present, because it's where you are. There are still people in 2023, which is a place/time that you were just previously at.

Imagine six pieces of very long string with one bead on each string. this is the timeline. The stones will move along the string as time progresses. But in 2018, when Thanos destroys the stones, he takes them out of a solid form, the stones are still there, just not usable.

To get a stone from before it was reduced to atoms, you must go to a time before it happened.

If you go to 2012 to get, say the mind stone, you have two options, branching the timeline to remove it, (Cut the string to slide the bead off) or you can stay with said infinity stone until your present day, (sliding the the bead off the end)

If you remove the infinity stone/bead from the timeline you have to cut the string, preventing that timeline from effecting the main stream.

These rules are completely invalidated by Cap in the ending however.

Not really. The movie alone implies Cap time-travelled to the past to live a life with Peggy Carter. With everything that was explained in the movie, it should be entirely implied that Cap lived a life with Peggy in an alternate timeline. The confusion in regards to whether or not Cap stayed in an alternate timeline stem from the writers, which said Cap stayed in the original timeline this whole time, but the directors corroborate the alternate timeline hypothesis, so it is not clear. That being said, Cap time-travelling to the original timeline and staying in as a loop is technically possible, if we consider the timeline loops presented by Agents of SHIELD for instance, where you can time travel within your own original timeline and spend your time there without changing the timeline itself, because you time-travelling to that point in time was meant to happen. Agents of SHIELD shows two forms of time travel, in the sense that there is one way that allows for the timeline to remain the same despite having time-travelled to the same timeline, and there is another way where you create an alternate timeline. So we could be looking at two different forms of time travel within the same movie. That being said, however, Cap staying as the secret lover of Peggy Carter for the entirety of the main MCU timeline sounds very odd when you look into the details.

Time travel is chaotic. But there will only ever be ONE of each infinity stone in its respective universe.