Unless it hits a duck, what goes up, must come down, somewhere.
Yes, it could be. In a rare case, it could cause death. The terminal velocity of a bullet can, unlike the low terminal velocity of a falling coin, be surprisingly rather high.
Yes. Just don't look up.
Due to air resistance, the speed of the bullet upon return must be less than 710m/s, a lot less. That helmet should, I would think, easily handle it, if your data is correct.