I have a Shaver 165; Nature's Comfort is the same basic design. I burned wood only the first two years; just started burning bit in it this year. It's not really an ideal appliance for burning coal IMO, for the reasons you state. The clinkers build up in the firebox with no good way of getting rid of them. And unless you limit the airflow, that fan forced draft gives the coal a lot more air than it needs. What I have been doing is burning a mix of coal and wood. I load wood on the sides, put a pile of coal on the grates and then some wood on top the coal. It seems to me that the burning wood helps the coal burn more completely, reducing the clinkers in size so they work down through the grate easier. Not saying this is the best or only way to burn coal in these boilers, so take this FWIW. I'm still experimenting.
I'm certainly using less wood. I've thought about putting a spin draft in the loading door since bit likes some over the fire air. I also wondered if natural draft would be better than fan forced. The last load of bit I got was $105 a ton, so even though my wood costs me very little,it's worth it to reduce the wood cutting a little. If I had to pay much more for the coal, I'm not sure it would be worth the trouble.
BTW Shaver is selling a shaker grate for coal that will replace the regular grate in my OWB. Don't know the price or if it will fit yours. You could check with them.