Yup. But it's at their discretion, and you can bet there will be political pressure from above to use that discretion to make everything that could be considered a firearm under the GCA, considered a firearm.
IIRC there are already firearms where ATF considers more than one component to be a firearm in and of itself, i.e. one complete gun is comprised of two or more parts that are legally "firearms" independently.
I was surprised they did as little with the new reg as it appears. It wasn't for lack of intent, but they discovered there are limits to how much further they can go without statutory changes.
Unless I missed it, NO.
The NPRM says they will designate one piece and grandfather in what is traditionally been the firearm on older guns.
So, what already has a serial number will be the firearm. Moving forward ATF will decide what the firearm is for new guns once a manufacturer submits it to them for review.