Ok, I'm back. After all this time and fiddling, it still goes into limp mode if the throttle is opened more than halfway. I just got done pulling the turbo and cleaning out the minor bit of soot built up in there. I'm guessing it's a nearly new unit, so it's not surprising there wasn't much soot. I also replaced the N75, MAF and lower boost hose from the turbo. While the turbo was out I deleted the EGR cooler, but left the unit itself attached to the plug. The turbo was my last hail Mary for a solution, so now I'm unsure how to proceed. Prior to pulling the turbo it was overboosting, but now it indicates that it's failing to reach boost specifications. I've been fiddling with VW, Mercedes and Cummins diesels for over thirty years, and this is the only one which has completely confounded me.
To add insult to injury, during our recent PNW heat wave, the car began to run hot when operated for any period beyond about fifteen minutes. The fan clutch didn't seem like it was locking up as I would expect given the heat, I had to endure running the heater on full in order to lower the temps. FML