@TheHondaBro I can't help you much with your problem, but just wanted to give some background on some of the ODO stuff.
When I was working at GM even us engine calibrators couldn't reprogram the gauge cluster mileage. That part so incredibly locked down it was crazy. There were a couple times where we had to reset the ODO in the gauge cluster and we had to aks the manufacturer for proprietary equipment to do it, and even then they did not want to give it to us. We had to give them tons of assurances that it would be kept on site and only very specific people could use it. I can totally see that they wouldn't even want dealerships to have that power - someone could have a nice side hustle reprogramming.
That said, at least on the Duramaxs the mileage is kept in several places - gauge cluster, ECU and TCU. That's one way we could check if a truck came in for warranty repair when the customer would say "it's completely stock" but in fact he had swapped out the ECU for a tuned one, and swapped back the OEM one after it blew up. We could check the mileages and see they didn't line up. (Also, the TCU keeps record of the max extimated input torque, so if it's like 1200ft-lb we knew something was up).