Did you try my suggestion of accelerating normally and the mashing it to see if the foot feed sending unit was faulty?
Ha. That’s a good possibility also. I don’t know how that works but would the previous owner have been driving around in that mode?Plugs and ignition parts to throttles, cooling, turbo waste-gates and such. What about the simple speed limiter enabled "MyKey" which was available in 2012. The OP description is an accurate description of a speed limiter. Runs quite normal to the set MPH, then does not accelerate any more. KM