We have a chiropractor across the street who is also a big into biking. I spent some time with him as I was getting numbness down both arms. Looks like I may have a couple issues with over/underdeveloped muscles in my shoulders, back, neck so am doing exercises to create a better balance. Also having a neck and shoulders massage helps. In addition to several stretching of the arms, much like you describe, I do an isometric several times a day - relaxed shoulders, sitting or standing nice and straight. I tuck my chin slightly and then concentrate on pushing my chin back as if I am forcing it to the back of my skull. This works the muscles between my shoulder blades to help strengthen my neck.
Finally when, riding I have to really remember to keep my shoulders relaxed and in "my pockets" and to keep my chin tucked as I tend to stretch my neck out and lead with my chin, putting to much pressure on my neck. give it a try and see if it helps. Rode Hills on Friday and had zero numbness for the first time all summer! good luck, I feel your pain