that is unknown for now, as DTV visa option is available in just a little over 6 months and with Thailand visas rules and options which are changing so often, it's impossible to tell for sure.
based on current experience of many people who want to stay in Thailand long-term and ever had ED Visa is that long term Visa's use to be rejected because of previously used ED Visa. So, people who are trying to apply even for one of the most expensive option like Elite visa getting it rejected and are stuck in the never ending circle of every year renewing ED visa.
real life of many people shows different result. You can try proving that immigration officers made a mistake. I understand your point, but without actual proof I doubt who is right here. My personal experience is telling me something else than you are explaining.
I checked and I assumed that extension is granted for 30 days since day when you visited immigration office to extend your stay. Not 30 days from the stamp date. Hearing the same from other people, that's why I am surprised to see you telling me something else.