Yes thank for pointing that out.
Yes one of the concern I forgot about is aiming. Would be hard, agreed, the rifle would need to be farther away from your face than usual.
As for the lense, it is a motocycle face shield, I'm sure it can wistand BBs.
For ventilation, one fan above the visor to push air down, one behind the neck for pushing air down. And some fogtech (I think that's how it's called) on the visor to prevent as much as possible fogging. As for hearing, either use microphone/headphones or I could cut a small piece of plastic near the ear, put some mesh and cloth and paint it like the rest of the helmet.
I guess it would work better in winter condition.