A pistol with a foregrip is an NFA weapon, as is a pistol with a designed shoulder stock. Accordingly, I think you would have to Form 1 a KAPOS conversion kit, pay your $200, get fingerprinted and local LEO sign-off (or register it to a trust), etc.
I haven’t looked closely into this.
You are correct. BUT the folding AR buffer tube configuration works quite well to increase contact points for greater stability and does not require a tax stamp/registry, so long as a vertical fore-grip is not added ... AFGs are okay though.