I am drafting a letter myself. I think we should put this fiasco into the broader context of the difficulty with which prospective immigrants encounter when tring to obtain permanent residency. The purpose is not just to criticize and hope to correct the "mistake" made by the State Department (DOS), because even it is "corrected", many applicants won't get their greencard for years to come, not to mention there is little chance the DOS will admit its fault. In addition, even the DOS "corrected" the mistake, many applicants will, rightfully so, feel unfair because it will disturb the order with which the applications are received and approved.
I think it would make more sense if you keep in mind the purpose of getting the greencard earlier, and push for pressure on the USCIS or even new legistation on immigration. This fiasco is just one of many examples of the failure of US immigration law and enforcement.
Enjoy your writing and let's fight!