Before you do all that work, I must tell you that the only coal that burns well is blackshack Anthrocite.
No smoke, no clinkers, 18-26 burn time. If you can't get Anthrocite, I'd stick to wood.
Also, if you burn a little coal with a lot of wood you actually just burn your wood faster.
I burn 80%coal 20% wood and that works great. When you burn coal, and the temp is warmer than 35 outside you have to keep your diff at. 3-5F for coal or the fire goes out.
Now for wood, most people keep it at 12-15 diff. So, little coal with more wood is hard to make it work, because you need two entirely different diffs. I burn 5 ton Anthrocite with 3 cords the entire season;for a 4500 sqft house with Dom water, keeping the house at 73. 5 ton Anthrocite normally is around $1500, which is still worth it. I pay $500 but that's not normal. So I'd make sure that you have easy access to Anthrocite. That is the only coal that would give me the 18-26hrs. I tried them all.
Nature has been very helpful to me. Call them.
I did two modification to my boiler that reduced my fuel use and increased Burn time.
A steel plate flat horizontal laying on the bricks under the flue to prevent heat and flames going up the stack, and a steel plate vertical against the stack standing on the fire bricks. Each are 26x12 1/4 thick.
Good luck.