Tom, that 4th link you posted is about the letter that has actually stirred all this schitt up!
General consensus is that the issue with the SB-15 on a shotgun in the first link pretty much has no bearing on AR pistols as it's a totally different platform.
As for the Blade arm rest that is talked about in the third link, that dipshit is trying to make money off the SB-15 concept and the prevalent attitude in the AR pistol community is that it's an overpriced piece of crap and will most likely disappear over time due to non interest and lack of sales. Another reason it won't sell very well is because a lot of AR pistol folks feel that by Martin Ewer submitting the Blade to the ATF for approval as an arm brace when it obviously doesn't do THAT job as well as the SB-15 and looks even more like a shoulder stock, he poked a beehive that should have been left alone! Yeah, this country was founded on free enterprise but Mr. Ewer hasn't endeared himself to the AR pistol community at all.
After reading all the conjecture and speculation I'm of the opinion that not much is going to change with the SB-15 until the ATF actually tries to prosecute someone for the missuse of it. As someone pointed out, if a person at a gun store or gun show picks up an SB-15 equipped AR pistol and shoulders it have they just made that pistol an SBR and are they now guilty of a felony? The ATF has really muddied the waters on this whole situation and until I see a court ruling I'm keeping my arm brace on my pistol.
Here's my personal take on this situation. If you build an AR pistol with the INTENT to shoot it as a pistol then by putting an SB-15 arm brace on it you are actually reinforcing that intent, as you have installed an approved device to make it easier to shoot that firearm in the manner it was designed to be shot, i.e. as a pistol.