Hi Doug, I'm still impressed that you can get Buckwheat to burn well in a hand fired stove.. usually Pea is the smallest size that will burn well with natural draft.
I suspect that with the smaller size coal, that the ash removal is very effective with your shaker grates,, instead of trying to grind up larger husks of burnt-out coal and large pieces of ash, the smaller pieces just fall through, this would keep the air passageways open and the fire burning better..
The lower burn rate and burn temperature is keeping the fire temp below the AFT, the Ash Fusion Temperature. If you burn the fire really hot, the ash will fuse into a big sheet of clinker, which blocks the airflow, and won't shake down.. This is what happened every week in my big handfed boiler..
Your Clayton firebox has nearly vertical walls, this helps it burn with out compacting the ash, which also creates clinkers..
I wish I was nearby,, I give you a few hundred pounds of pea coal and you could see what the furnace burned like with a slightly larger coal..
I'm surprised that the draft was the same at the flue compared to the inlet of the ashpan, were you useing the long probe throught the ashpan spinner?? that would make more sense.
Take care,, Greg L