After 2 months with arm and back shoulder pains and nigths w\o sleep I found a way to sleep well. (side position).
1. Lay my body from armpit to feet on a mattress.
2. Leave my shoulder and arm literally free in a narrow valley. (valley deep enough not to touch the groung in a way to produce a force back to the body and spine and at the same time lay confortably on the ground).
3. Lay my head on 2 pillows (one harder bellow and one softer above) approximatelly 4 inches higher than the mattress.
Pillow and mattress are appart of each other only enough to leave space for my shoulder.
It worked for me. I seems that my cervical stays horizontally aligned and does not receive any side force coming from my shoulder.
I will try again tonight and let you know. Hopefully well again.