A typical wood fire burns hot and fast,, unless you starve it for combustion air, then it smolters and smokes, and makes creosote.
Your first burn with your Bit coal sounds like it did OK.. you recognized the coal bridging and you broke it up,, so air could get through the coalbed and burn.
The next step will be to slow the heat down as it leaves the fire and travels up the chimney.
Do you have a thermometer on or stuck into the flue pipe?? Any MPD [manual fluePipe Dampers] ??
I think your US stove furnace has a sliding 'bypass' in the top of the combustion chamber, it should be positioned to route the hot exhaust around and keep it in the steel box as long as possible..
The US stove products make a much better wood burner than a coal burner.. with coal, you need to extract as much heat as you can from the exhaust gasses then let it up the flue.. with wood, you need access to clean out the creosote, which means a very simple box for the combustion/heat exchanger..
I'd try it again, Break up the coal into small as possible chunks after it has stuck together, Make the fire as deep as possible,, mound it up in the center, with the sides at the top of the firebrick..
ONly use under fire air from the ashpan air inlet.. shut off the over fire air through the loading door after about 20-30 minutes, or once the initial yellow /smokey flames have turned blue.
close the MPD if you have one, this will help slow the exhaust leaving the steel box
Slow the fire down, and run the circulation fan all the way down the 85* or so.. get the heat off the steel box, and into the room.
You can put a wood split or 4" round down the middle of the coal, then load up coal on each side, this will leave an open path that burns away, leaving a combustion air pathway through the coal..
hope this helps..