Valeris probably wasn't grabbed or otherwise decided upon by Kirk, as she was Cartwright's inside gal... The dramatically likely turn of events here would be that the previous helmsman met with a force majeure
that wasn't as innocent as it sounded.
Although Valeris could also simply have been dictated upon Kirk, or infiltrated into Spock's inner circle long in advance (Spock was
in the process of betraying Starfleet to the Klingons, as far as Cartwright was concerned).
James "Nixon" Kirk was quite probably given the clearance to kick out the ship's bridge officers, so that he and he alone could show the Klingons the hot end of the phasers / act as the perfect scapegoat. That as such wouldn't tell us whether the ship was until then crewed for a standard exploration/defense mission, serving as a training ship, or cold-rotting in long term storage - or even being displayed as a museum piece.