Hmm, I remember more about WC 2 than any same person would care to admit
.
Minx was, indeed promoted before we met her, and frankly, I don't see anything strange about that. After all, Jazz was also a traitor, and he was a Major. I don't see why Minx shouldn't be Captain - hell, you don't know how long she's been flying.
As for the theft. Pilots normally have access to the flight deck. Keep in mind that with Jazz gone, everybody thought the Concordia was spy-free. Anyway, Minx set up an explosive device on the flight deck. Then she went and told Sparks that she was doing the patrol today (the Concordia did not have the computer system that appeared in WC 3). Sparks had no reason not to believe her, so she cleared her Morningstar.
Then Talon and Crossbones came down onto the deck. They immediately realised what's up, but it was too late - the flight deck went up in flames. Talon and Sparks were wounded, and Crossbones died. By the time the flight deck was back in operation, Minx was already too far away.
And yes, later on Jazz left Minx and everybody else stuck on the space station while he fled in the Morningstar. Minx sent a message from a failing life-pod, and died. Then we went after Jazz, and had the satisfaction of seeing him die
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There. That's probably more then you ever wanted to know
.