There is a lot out there on this subject. Search.
You will see a lot of arguments about building pistols off stripped lowers, DROS, building a pistol off a virgin "rifle" lower legal by ATF standards, etc
I'll make it easy: Look into 80% lowers.
To answer some of your questions
An AR Pistol would not be on the DOJ Roster. That is why you see things about SSE.
Yes it would need a magazine lock and 10 round magazines because it would be a pistol that has a magazine outside the pistol grip.
12276.1. (a) Notwithstanding Section 12276, "assault weapon" shall also mean any of the following:
(4) A semiautomatic pistol that has the capacity to accept a detachable magazine and any one of the following:
(A) A threaded barrel, capable of accepting a flash suppressor, forward handgrip, or silencer.
(B) A second handgrip.
(C) A shroud that is attached to, or partially or completely encircles, the barrel that allows the bearer to fire the weapon without burning his or her hand, except a slide that encloses the barrel.
(D) The capacity to accept a detachable magazine at some location outside of the pistol grip.
(5) A semiautomatic pistol with a fixed magazine that has the capacity to accept more than 10 rounds.