Great write up!
I wonder if this could relate to some instances with the infamous P0741 problem. I think there is a seperate selenoid that controls the torque converter. In my case the car shifts fine, no noticable slipping, etc. Which leads me to believe the torque converter is functioning correctly. Throws the code with a bump when 5th gear is underpressure (such as hills on the highway). To me it seems as if something is not properly engaging and locking up the torque converter.
This too would be a great money saver if possible bc through all my research everyone takes it to places that just replace the torque converter for lots of $$$
Thanks for the insight!