Not necessarily. If Darci-A went back to 1999 and then stayed there, she could age twenty years and then go back and save Darci-B and the Main Character. Don't think that's what happened, but it's possible.
Not really,
because this time with the Mc was the first time Darci went back in time. Another version of her couldn't have gone back before the version of her with Mc if it is fixed to X-number of years in a parallel sense. So, to travel back to before herself and wait would also mean that she was traveling back to different point in time just as much as if she had found a way to travel relatively free though time. The two Darcis means that time travel is more fluid than the early time travel moments made it seem in at least some way. The fixed 20 years or so that everyone had previously been assuming must be false, though the true way it works is still unknown. However it plays out though, the door is open to all manner of time travel shenanigans.
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As an aside:
At the end of the day, I honestly hope it just goes with the basic Looper explanation, "I don't want to talk about time travel because if we start talking about it then we're going to be here all day talking about it, making diagrams with straws."
Basically, just let it happen how it does and then say, "It's all fiction, it works how I (Jestur) wants it to, and I don't care about perceived paradoxes and stuff, it just works this way, because I invented it and am the God of this fictional universe."
Not that theory craft isn't fun, but at the end of the day I just want to the story to be satisfying even if that means some "rules" of time travel that other people made up for their version of the fiction are "broken." Similar to when people act like there are real rules to things like Vampires and then get mad when a vampire story comes along and "breaks" them. Like, they aren't real, the rules can be whatever the creator wants them to be, let alone that many establish franchise have different rules.
But I'm rambling again,
I'll stop now.
