I don't have a Harman. But I do use an L shaped poking tool to work out ash in places where the the glow looks dim underneath after shaking. I go in thru the ash pan door with it and jab up thru the grates. I also use a slicing tool under the grates to slice along the length between the grates and grate frame.
As for CO while it burns down, keep an excessive amount of secondary air going in and/or primary air while it's using up the last bit of coal. I highly recommend everyone with a coal burning appliance to install a permanent manometer so you can see if the draft is trending towards failure.
Good job with the CO detectors, there is no such thing as too safe.
Good luck keep us posted