You can make a mix of 50/50 white vinegar, and water, completely submerge the grates for about three days or less. I suspect you may be making a mountain out of a mole hill, and being overly concerned about the amount of rust. in light of what you have done so far, slap on some oil and season it again I am willing to bet they will be good to go, if they are just reddish rusty looking and not deeply crusted with flaking rust. I would be more concerned about the water in the cooker its self, and how you can stop it from getting in in the first place.
Lg Spicewine on trailer
Weber charcoal grill
2 ugly drums, semi retired
Bbq toolbox, formerly Chargrill smoker
My wife has gas, its a Brinkman.