I would vote for underhoned. White and gray hairs are chemically different. Stronger and harder than "typical" beard hair. Thats for the cheeks. The neck is probably getting irritated because of the same condition, but not for the same reason. The neck skin has a lot of microfolds that the blade is probably abrading against. I would pull the neck super taut and go lightly. The microfolds are probably getting shaved at too great an angle.

So, hone the razor a little more, and on your neck stretch it out much more and use a very low angle.

Try that.