Here are some thoughts. My guess is that the death certificate will not be enough for the company to begin paying royalties. Typically, the division order analyst will require that a probate be filed. A probate for your grandfather and mother can be handled in a single filing to reduce costs. The probate order will include a determination of death for your grandfather. The property will be divided according to the Wills or intestate succession. So if either or both did not have a Will it should not be a problem. This sounds like a matter that can be handled on a flat fee.
I am surprised that Oklahoma cannot locate a death certificate, is it possible that he died in another state. Perhaps while traveling?