You'd have to be a pretty good shot to bring down an airplane in flight with one of thses -- I don't think that's what they had in mind. But if anyone can do it, I hope they're on our side.
Exactly and the Army isn't claiming you can. Notice the reference is to "Parked" aircraft. Even then you'd have to be using incendiary rounds and get a hit on a partially full fuel tank. Or if you have a bit of time, you could take out the engines one at a time, with several shots to each, to be sure of hitting something not easily repaired.
Barrett made a similar model designed specifically for anti-aircraft use. Didn't make many; IIRC, not very effective.
Yes, but helicopters might be considerably easier.