- Stock: as long as you have a legal "useful purpose" for the stock other than making an NFA firearm, you're okay. Meaning, you could have the tube for it on the pistol, and the stock would have a legal purpose if you also possess an installable long barrel, or if you have another rifle with the UBR setup, as an extra stock would just be a legal spare part.
However, if all you owned was an AR pistol, possessing any stock would be unwise, and one made to attach to the type buffer tube you have would certainly be making an NFA firearm under the ruling (parts do not have to actually be assembled to have "made" one).
Anyway, the test is not whether you can make something illegal with anything you have, but rather whether you can also make something legal with it.
- 26": This is only relative to an AR pistol in that if indeed it is 26" or more in legal overall length, you may use a vertical forward grip, at which point it becomes a legal "firearm", and magically reverts to a "handgun" when removed. Note that muzzle device only counts toward OAL if permanently affixed.
- OS