I have diagonally growing neck hair as well. It's a complete pain.

It was a difficult period, but I have taught myself (slowly) to do an outside to inside pass with my opposite hands. So, (i'm left handed), if I'm using my dominant hand to shave my left cheek and WTG pass on the left side of my neck, I'm ALSO using it for an ATG pass on the right side of my neck. And vice versa for my right/non dominant hand.

I need to reiterate that it was a slow process, with 100 percent focus. The angle for that portion of my face coupled with a portion done with my non dominant hand was frightening at first, but it works well.

Good luck, and take it a little at a time. If it doesnt feel right, stop.

Cheers,
Maxi