They're functionally equivalent to what Miller was in trouble for having, and which SCOTUS ostensibly couldn't see an obvious military use for.
I'd go so far as to say: in observing that the defense didn't show up to court, and recognizing the serious relevance of the case, SCOTUS was looking for a way to make the case go away without making a binding ruling - as to do so would be unfair to both sides. Better to a relevant yet easily-answered question which sends the case back thru the system, allowing Miller to pursue his case (if he ever showed up (difficult, being dead)). It's not unusual for courts to seek any way to weasel out of making a ruling (even in Parker, only one plaintiff was granted a full ruling).
No. They have been declared suitable for military use. So has the M79 and the M249 SAW. That's what's relevant.
For you to say shotgun "A" is allowed therefore shotgun "B" should be allowed is ludicrous. If the U.S. Army thought there was merit to a 17" shotgun they would have told Colt to build one. They didn't. They told Colt to build it 20".
(but ... but ... what about 19.8" robertpaulsen?)