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.
MY DEFINITION OF AN OPINION.
I can only give credence in facts I believe factual. If those facts are wrong, and are proven as such by facts, I must only give those facts credence, if I believe them as factual.
When the going gets tough, I take more fiber.