The reason those grates are badly warped is that the ash pan was not emptied often enough.. a red hot fire can sit on the grates without harming or warping the grates. BUT if the ashes in the ash pan are deep enough to touch the bottom of the grates, then the cool air coming in from under the grates is blocked, there is not cooling air circulating over the bottom of the grates,and they overheat and sag..
So, keep the ash pan emptied every 12 hours or so, this will depend on the burn rate you need for your weather.. Once you figure out a routine you will be very happy with that stove..
Get rid of the manual had damper in the chimney flue pipe.. install a Fields Barometric damper, type RC in the flue pipe.. This will make keeping a fire burning for the whole season much easier.. Many coal berners take pride in starting the fire once for the whole season,, and it burns till they let it go out in the spring..
What is your chimney made of, how tall is it, and have you had it cleaned since you burn wood too??
Once you get your grates replaced, read about starting and maintaining the fire, there are many many threads about those subjects on the first few pages of the Hand Feed forum..
Hope this helps