I have one other new theory/question to run by you guys...
I had speculated at the beginning of the thread maybe the 5 degree increase being seen at most ducts was due to attic infiltration in the supply plenum over the AHU. However, what is the chance that instead there may be infiltration at each individual duct where it connects to the ceiling vents? If the flex ducts were not properly fastened/sealed at time of installation, might this explain how attic air could be infiltrating? I do know that one of my neighbors' HVAC company made this exact finding and she paid to have all the ducts properly fastened.
Thoughts?
If I were to go dig through the blow-in insulation and inspect a few vents, what would clue me in as to whether I need to call for inspection.... should I see tape, mastic, zip-ties, something else?