Yes It is a good idea but you could do something better the way It iy my wall was It used a white fabric and It hung it up out side and through ballons at it and ever wall It did something else fliging paint at the fabric and useing a duster to to make it look really cool. So you could get some fabric and do something else with ever time.
I would think if you have permission
Yes,but stand 5 feet away from the wall or else you will have a lot of holes.
Maybe. You would have get some special made paintballs though for painting, and you might want to turn your velocity down a lot, so you don't end up with a lot of holes in your wall!