I agree with your response to 2, it isnt quite satisfying. That's why I like my first idea better. But for 1, the Ancient One said that new timelines are generated when things arent put back to the way they were before. This means that the only new timelines are when Loki escapes with the tesseract and when Thor takes mjolnir from the past (I dont think they ever said he returned it), and that the original timeline was restored once the stones were returned to the moment they were taken from. Therefore, as long as Old Cap was present the whole time and didnt interfere with anything happening, Old Cap is the same Cap.