ATF says 26" OAL enters the "firearm " world and forward grips are allowed....better measure right thoughIf that AR started its life as a designated AR pistol my understanding is no forward grips of any kind, and no buttstock of any kind, including the shockwave and Sig brace regardless of OAL. Been doing a lot of research as I'm about to build an AR pistol myself.
once the grip is installed it is no longer considered a pistol so anything that applies to pistols like concealment for instance is no longer valid
I think it depends on where you live for the designated pistol thing. I am not sure of the states but some say those rules you stated .
Here when a lower is purchased it is checked as other in the box so you can go either way with them
no butt stock of any kind is true.....but you can add braces all day. the brace is an accessory too the pistol
Federally, and pistol can be built into a rifle but cant be returned to pistol state once done