I also think Greg and Highlander are on the right track. Those grates look like they've been subjected to sustained intense heat with improper or uneven airflow underneath.
There is a cleanout trap under the grates- looking down from the inside of the glass, there should be a lever-looking thing pointing to one side, underneath the grate holder. Can you swivel it forward to open the dump? Even with my combustion fan running full time, I kill the power for a short time every month(ish), cool the stove enough to reach in from the front with a fireplace poker, and open that slide to clean out some fine ash into the pan. If allowed to accumulate, I could picture that fly ash getting compacted and messing up the airflow.
If you still have the grates out, you should be able to look right down through the grate holder and see the ashpan if the slide is open.
Just to cover more bases, any idea what the stack temps were before the baro damper and/or surface temps of the stove when running? I have to re-read this thread, but only 45,000 btu's, an uncontrollable draft, and grates burned out like that don't add up (I'm no math whiz either, by the way...
) Unless the draft is pulling air through partially blocked grates, making for hotspots at wherever the fire can suck air, but the combustion blower is somehow not feeding air?? Maybe??