Rest assured, my question is entirely Endgame spoiler-free unless you hover over the hidden quotes. Probably best not to read on.
Time travel is fickle, and Endgame's plot is centered entirely around it. Conventional time travel has a golden rule: change the past and you change the future. I am aware of what is said at the start of the film: (quoting Banner/Stark? from memory) "whatever happens in the past the future remains the same". This may explain how reality remains OK even after the un-snap with the vanished half suddenly coming back 5 years later, as the future goes on normally as if the vanished had really vanished for the last 5 years and only reappeared again without having to fill in the 5 year time gap.
This begs the question, however: How is Nebula still alive and kicking at the end of the movie, after she kills (literally shooting a hole through) her past self? Logically speaking, shouldn't killing her own past self erase her from existence, completely, in the future?