Update:
OK! So, I followed your advice (matched Jarom's torch exactly, got a PSI regulator, played with the air tuning, etc.) and it works now!! Here are the changes:
Found the correct bell reducer at Lowe's in galvanized (that's a bad idea, I know, but we will make sure it doesn't get hot)
Replaced the pipe nipple to full 8" length
Got a 0-20 PSI regulator (It's weaker than recommended, but it works)
Drilled holes in the bell reducer to allow greater airflow. We noticed that when we blew air into the forge with a copper tube, it got WAY hotter.
As a result, the torch heated the metal to a yellow color, which I think is forge-welding temperature(?), and made it really easy to work with. The bricks do take some time to heat up, but after 10-ish minutes at 10-15 PSI they turn to a bright cherry-red color. Now that this is solved, we will remove the top layer of bricks, drill a hole in the two remaining roof bricks, and weld the nipple to a piece of metal to go across the top and finish the frame.
Thank you so much for the help and feedback!
~ Royal