Fitting the undercollar

I'm having the toughest time ever fitting the collar. This is the time I feel the need for in-person coaching the most. Simply put, there are very heavy creases around the neckhole.

Was it due to an overly small neckhole? Was it due to an overly high neckhole? Is it a problem to be corrected from the upper center back seam, the shoulder seam, or does it entail merely scooping out the neckhole?

I drew a larger neckhole and rebasted the undercollar. The problem was ameliorated somewhat, but the creases were still very heavy and ugly (about 3/4 inch of excess). I then drew an even larger neckhole -- it appeared to me comically large -- and let the neckhole creep into the shoulder seam by about 3/8". The creases decreased in severity further. Then I though maybe the problem was the seam allowance -- all that cloth obstructing the neckhole from lying flat, so I snipped into the seam allowance at the neckhole and loosened the stitches at the CB seam and shoulder seam. Then I adjusted the shoulder seam, drawing in about 1/2" of the back panel near the neckhole to nothing at the shoulder tips.

After all that trial and error, the results are much better but still far from perfect:

The white arrows point to the areas of concern.

What I'll try next is to take in the upper CB seam by about 1/4" on the double at the neckhole to nothing 2-3" below it..