I liked the "new" characters - Vol needs to make a comeback at some point, even if Carolan suddenly lost all of her coolness (imho) in the jump between the two sections of the book, and we never really got the fleshed out character for Kat that we really should have recieved. It's these that make me think that perhaps we were intended to have more adventures with the crew.
I definitely want to use Kat again in future. No plans for Vol, though anybody else can use him. Carolan... who knows?
Yes. In fact, in the original pitch he was Challenger's Captain (when it was just a Geordi book, with no Scotty et al), and so that was always going to happen. The trick was to think of a way to deal with Scotty being above him...