If the grates will install upside down, even without the actuator rod hooked up, later in the season, I'd try to get a good hot fire going on the grates, with a close to full ashpan,, and see if they sag back some.. If you can see the grates through the ashpan door, you might be able to gauge the amount of sag or 'hump [when upside down] the grates have..
So you could use the stove as your foundry,, get it hot, look at the grates, if they are red, either let 'em sag or GENTLY ! from above push down on each grate with a strong poker or crowbar. If gravity sagged them it shouldn't take too much pressure.
Since you are probably going to break them anyway, unless you find a foundry furnace to put them in, ad slwoly cool them down in,, it will be a good experiment and you will be repeating the conditions of the original sagging.
just a thought..