You have very impressively examined your neural tension for us (sitting and flexing your neck forward).
Nerves DONT stretch... they glide, and if you try to stretch them it will be very sore... What it sounds like to me is that somewhere along the pathway of your nerve (likely the lumbar spine) there is an adherence to the nerve which is stopped the nerve from gliding properly. This could be from lots of reasons, most likely an old back injury that is now healed.
I would suggest you see a physio to help guide you through this, but basically you want to try to break that adherance... you won't like it... you go into the painful stretch and then bob your head up and down, gently tugging on the nerve trying to break the adherance... a bit like trying to floss the nerve. I say this because you can work from both sides, so you can also move the nerve by flexing the foot. The thing is there may be some manual therapy techniques a physio can do for you around the back or possibly the hip to aid the loosening of this adherance... So I suggest you get some professional help to see if you can get better faster.
Happy Stretchin'
Puddin'