An electric fan on a derby car works the same way a propeller does though right? You don’t need the sail at all. It works by pushing against the surrounding medium. This seems different than a laser in a vacuum.
Not really. If a fan was able to blow air in a vacuum it would produce thrust as well. Basically a rocket does that. It's just blowing gasses out the back.
It's Newton's law of motion, "every action has an equal and opposite reaction".
If you attach a fan to a derby car and point it backwards, the car will move forwards. If you redirect the fan so it's pointing forward, the car will move backwards. If you put a sail in the way, the car will move backwards less efficiently. If you make a perfect sail that blocks all the air from the fan, it will not move at all.
It should push the fan and the sail apart, which could result in a structural failure.
The more realistic outcome would be that the sail did not disperse the air with perfect efficiency, and the craft would just move backwards.
What you’re describing would require redirecting the flow of air backwards, like a curved exhaust. Which is really just a less efficient version of turning the fan around.