Yes. United States law recognizes lower receivers as the definitive part of a “firearm” (the part that loads magazines and houses the trigger). This is why Lower Receivers have serial numbers and Upper Receivers do not. This means that SBR Upper Receivers, stripped or assembled, are perfectly legal to own as long as they are not attached to or stored with a non-NFA registered lower receiver.
No. Since the Upper receiver is not considered a firearm by definition, they do not require transfer through a FFL holder.
No. Assembled rifles with barrels shorter than 16″ are Short-Barreled Rifles (SBR) and regulated by the National Firearms Act. Only Law Enforcement/Military agencies and authorized FFL dealers may order or register a Lower Receiver as an SBR.
Yes. Lower Receivers purchased as Pistols are registered as such by serial numbers and are not considered rifles. Compatible SBR Upper Receivers may be used with these weapons.