They can take a coin out of their pocket and release it in front of them. If it stays stationary, relative to them, the ship is not accelerating. They feel weightless.
All directions are equivalent, so up and down have no meaning.
If they are sitting in seats and facing B, they will feel pushed against the back of their seats. If they're not in a chair or anything, the wall at the end of the ship away from B will come towards them, and they will feel pushed by it. Now the ship slows down, which is acceleration in the opposite direction. From the side, we can't see the laser. All the laser light is going towards B, and there is nothing in space to scatter any of the light to our eyes.