My buddy and I fixed the Buick. Well, we fixed the symptom. No idea what the heck was causing the issue itself. Fuel trims were weird and the car was cruising at 15 degrees of timing when it should have had 30+. Disabled the ECUs torque protection by zero-ing out the appropriate table and... it's fine now. For some reason the powertrain was convinced I was going to destroy it at 10% load on the highway with the cruise control on. For the life of us we can't find any faulty sensor readings, so I'm leaning towards the ECU itself being faulty. At any rate, went from 15mpg to 24mpg indicated with flawless fuel trims now and far more appropriate timing.