First, I'm no lawyer, nor do I play one on TV or the Internet.
I BELIEVE, that having a single short barrelled upper, along with a SBR registered receiver, or a pistol receiver, would be an adequate defense against prosecution if other COMPLETE ARs are owned.
Where you COULD run into trouble, is having more SBR uppers, than LEGAL receivers (SBR Registered, or pistol) for them, ALONG with either complete ARs or complete RIFLE lowers.
I hope I'm making this clear. In other words, if you want to have 3 SBR uppers, you need to have three SBR registered lowers or pistol receivers.
While you may claim that the SBR uppers are ONLY for the SBR Registered lower, or pistol, having other AR rifles or lowers MAY cloud the issue enough to cause you legal troubles. And while it MAY in the end prove to be legal, attorney's fees would certainly pay for registering multiple lowers.
MAYBE you could get away with listing barrel length on the Form 1 as 7.5", 10.5", 14.5" etc.. or "multiple', as a pre-emptive defense.
Call me paranoid, but if I'm wrong, I lose very little and when it comes to jail, I like to stay on the overly cautious side of the law.
Lonny