You can read all the "letters" you want, they aren't law. Just the interpretation of the rules at the time the letter was written, by the person signing the letter, regarding the specific case or question asked. They can change before the ink is dry.
Pistol buffer tube vs rifle stock isn't cut and dried. There's no clear definition of the line between the two.
The hint has been that if you use the buffer tube, regardless of it's length or shape, like a rifle stock (e.g., put it to your shoulder) then the ATF and the AG might prosecute you. MIGHT. A lot depends on the time of month, whether you're a thorn in their side and whether you're part of the ruling elite or not.
Given the W-I-D-E discretion the AG seems to have regarding whom to indict, based on whose side they're on, I wouldn't tread to far away from a standard, stock buffer tube.
Make all the mods you want. I just wouldn't take it out in public and I sure as heck wouldn't show it off.