1. Yes
2. No, HBAR rules do not apply to SBR's
3. Once it's an SBR it can have any length barrel attached as long as it doesn't make the OAL less than 29in
4. No
5. No, but suppressors do count as flash hiders. So they cannot be used in conjunction with a folding stock or grenade launcher
6. No...
This. I just dealt with an NFA transfer after a death and the item was not in a trust. Huge PIA. I'd also steer clear of the silencershop and other "single shot" trusts as you cannot update them after creation. I've used NationalGunTrust for one and the process was easy and cheap. They also...
With SBR's and AR's outside of 5.56 or .223, the HBAR rule doesn't not apply. Someone correct me if I'm wrong but a telescoping stock does not count as a "feature". Only folding stocks do.
I had a similar experience with the blazer 147gr. Look into the Federal syntech 150gr. It's all I use now when shooting suppressed 9mm. It has a polymer jacket, cleanest stuff I've used.
I'd assume it's one of those things that no one would ever notice or care until the rifle is used in an unlawful way. Then it's just another charge they can slap you with.
https://www.atf.gov/firearms/docs/form/application-transport-interstate-or-temporarily-export-certain-nfa-firearms-atf
I fill them out yearly so I can take my SBR's across state lines