@Lex
Seeing the points you've brought up, and also considering that they have committed to making smaller details matter (e.g. when they said brighter/whiter clothes will be more visually obvious to zombies), it is entirely possible that the devs have added such attention to detail. In earlier builds (early access) pistol suppressors would entirely mask the gunshots from zombie detection, as if they'd not existed, and rifle suppressors acted pretty much the same as current builds.

Update 2: Correction, all hat-type items will prevent the NVGs from activating, except of course the tactical helmet with NVGs mounted.

Update: This bug appears to effect all helmets, upon further inspection.