Hi CM, one thing you may experience with your firebox design,, which is pretty much the same design I used to build my 'Big Bertha' SS boiler. the 'V' shaped firebricks act as a funnel or concentrator of the ashes.. this is fine when the ashes are dead and cool.. but if you get really hot coal ash being concentrated or funneled down to the narrower grate area, the hot ash and coal is forced together and this creates clinkers..
Your grates are rocker grates, and these are good at grinding up hard ash.. My grates in my 'BB' boiler are slider grates.. they don't tip or rock,, so they only shave the bottom of the ashes off,, this didn't do a great job.. So after a week of hot, hard burning in the dead of winter, I developed a sheet of clinker on top of the grate,, and it would block the airflow, and I'd have to let the fire go out and clean out the firebox, and start over again. This got 'old' real fast.
The 'V' shaped firebrick are one of the 'issues' with the OWB boilers, the US Stove units, and other combo wood/coal appliances.. the 'V' works great with wood ashes. but does not work well with coal.. But it can be worked around.. it's just more work every week. A made for coal firebox will have vertical or nearly vertical sides and the entire bottom made of rocker/grinding shaker grates.. this allows the fire to burn vertically down, without concentrating the ashes and creating clinkers..
With white ash coal, and not burnt too hot, clinkers are not often created, so you might do well with your OWB with the right coal and techniques..
Take care,, Thanks for sharing.