paraesthesia of the R arm in the C6 distribution which came on "'overnight"" in Oct 2005 and has not changed since.
Perhaps this suggests some neck involvement in the lower cervical spine, or perhaps slightly more distally into the axilla region. I also never forget possibilities of DVT in the brachial artery as I have in fact had one of these in the past. This however often presents with associated mottling of the skin in the affected arm.
It is aggravated by Cx extension and relieved by flexion and he does not have it in neutral.
OK, so you have more now to implicate the neck. Is this extension in sitting or in supine with the weight removed. i.e. is is affected by loaded joint movement or by the movement itself? This may guide more to soft tissue vs. joint approaches to Rx.
He has very mildly limited Cx rotation L and R and otherwise full Cx ROM.
OK, but it is the lower cervical vertebrae that reduce end of range movement. Is he protracted in the neck? Does retraction ease his symptoms when provoked? Try retraction in supine with you holding the head to create an unloading effect.
He has no pain or change in strength in the R arm.
To have a true nerve root compromise you must be able to show motor and sensory loss. Normal strength suggests less probability of nerve root involvement.
I have also tried Mckenzie techniques involving passive Cx retraction/manual traction combined with extension which he does not have symptoms with throughout the action but it returns immediately on retesting of ext in sitting.
Good work. You are onto something here. Weight bearing is an issue, unstable disc perhaps? An Wikipedia reference-linkMRI probably will be positive as it is in almost everyone at the lower levels. You could try to inhibit the upper fibres of trapezius with taping techniques to assist the retracted position.

If you know any mulligan techniques (NAGS and SNAGS) you could try this in the retracted position, initially in supine and later with the patient actively retracting while you do MWM's etc in sitting. Best of luck and let us know how things get on. 8o