I know the Ballista has to be parked in order to fire the torpedoes, but will the gunner seat (the one that fires the missiles right now) eventually be able to swap between the torpedoes and S2 Gatling gun and fire it while the vehicle is moving? That doesn't seem overpowered and could come in extremely handy (and seems common sense).
There is no plan for this at this moment. The person manning the missile turret (or gunner seat) has very specific control over that turret only. The driver of the vehicle can drive around and fire the Gatling turret if they choose to.