Nebula crossing with her past self was shortly explained as the two "sharing the same network".
I think it makes sense enough. She is connected to what would be some sort of galactic internet and each Nebula would have the same IP address.
A Black Widow movie should definitely be a prequel. Her life before the Avengers has always been a mystery and should have a lot of room for stories. There's hints that she knew the Winter Soldier, so that could be a thing too.
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