Just thought I would chime in and offer a solution - I recently purchased a 13.5t D3, and had the same issue - the only way to adjust the timing was to take off the end bell and make the adjustment. The center ring would not turn at all. A real PITA.
I took the motor apart, and used 600 w/d sandpaper on the edge of the center plate to take it down just a hair until it would rotate freely in the motor end plate. Did that, put it back together, and *poof*, I can adjust timing now without any issues.
The amount I took off was *very* small, and it only took a few minutes. If you do this, just make sure you do it evenly. I held on to the plate, and dragged/rotated it across the sandpaper. Rotate the plate, and do it again. Went around the plate about 3-4 times, checking the fit. You'll know when you get there, as it will turn smoothly once you remove enough material.
T4 2014, 10r5.1 (F/S), 12r5.2 (F/S)
CRC Xti WC/GenX-10