Glad to hear the secondary system worked. I have a piece of stainless I may try. Gotta get a hole saw and bite the bullet and cut the hole. I have secondary air that enters above the fire through a draft slide in the load door, but it just isn't preheated enough except during the hottest part of the burn cycle.
My grates are working fine. I do get a few chunks of hard ash, an occasional clinker, and a few rocks, but I can go for several weeks or more before I clean the grates. Up to this point, I've been killing the fire every week or so to make some mods to the stove, so I've taken the opportunity to clean it then. Seems like lately I've been into some better coal, so I'm not getting much on the grates. Our western bituminous is excellent burning. No swelling or bridging that I can see, no crust to break. Just lots of volatiles at first, then a nice clean burn.
Grates have not warped or corroded at all in 3 months of use. I did not use epoxy-coated rebar. The only problem I see with my grates is that some of the chunkier ash tends to "ride" on the grates even when shaken, mostly the ash that's closest to the grate axles. The ash near the edges of the grates (furthest away from each axle) falls through fine. Might be better to make the fingers of the grates "mesh" together, that is offset the shorter pieces of rebar on every-other grate. That might tend to mix things up a little better. If that doesn't make sense, let me know. Also, due to some ash funnels I placed below the grates to better catch ash in my pan, I have relocated the connecting pin system for the grates, and I now use the center truss to connect the grates together. It works fine.
Good luck. Let us know how the grate rebuild goes when you get to it.