From what I understand, the part of an AR-15 that is considered the firearm is the lower receiver. That being said, it would seem to me like the upper is not a firearm in and of itself. So, can I purchase a complete upper receiver with a threaded muzzle break online and have it sent directly to a gunsmith in New Jersey for welding?

Correct, the lower is the firearm w/serial number that needs to be sent to your FFL and then transferred to you. You can have the rest of the rifle components delivered directly to your home.

 

Some manufacturers (ie Stag) also make compliant uppers that have the muzzle brake/ comp already pinned.