Person012345, what are you talking about?.. There was a point about propulsion drive, not a gravity maneuvre or light sail.
Surely, momentum conservation law is valid on close systems only, so if you want to use planet or star, then you must include them in your momentum equation.
But we are talking about NP propulsion drive (or another effective drive, not a cheap non-fiction one).
And NP propulsion drive is tossing plasma back, as I say. From the back of the ship, surely, not from the head. And it's how it works. Open drive camera - yes, but the same principle of tossing propulsion mass back, somehow or other.