"Could the regulator mixer be defective out of the box?"
Not really that likely. Be careful of your terminology in describing things. Mixer is the part that is sometimes called a carburetor.
Carburetor is incorrect because it is metering a gaseous fuel instead of atomizing a liquid fuel.
If you did the lean out test properly by spraying an alternate fuel into the mixer intake when the stalling began.....and you noticed an improvement in the running while spraying.........then fuel deficiency is the culprit, not ignition.
Not much of a practical way to actually test the regulator/vaporizer for fuel flow volume.
I wonder if 1/2" copper is large enough to operate the J model secondary valve in this application.
If you could substitute a larger diameter vapor hose from the regulator/vaporizer to the mixer that would solve that question.