Hi all,
Thanks for your thoughts and advice. I'm 16 days post-op now and I just want to follow-up on this topic for future inquirers.
Though internal tugs and throbs remain, this shoulder pain has pretty much disappeared and in retrospect, I think it's because I was cramping the nerves and muscles in my back and shoulders in an attempt to immobilize my chest muscles and not strain the stitches. However, at my first follow-up appointment with my GP she told me to start doing "shoulder rolls" and other exercises to try to get better circulation at the incision sites because they weren't healing as quickly as they should be. I found some gentle post-mastectomy exercises for cancer patients online and started doing a full set once/day, which helped a lot of things: my confidence, first of all, that I wasn't going to ruin the surgeon's work by moving my arms; my range of movement so I could start reaching for things at shoulder- and then head-height and above; reduced inflammation of the underlying tissue; better healing at the incisions; and the disappearance of that awful burning nerve pain. I wish I had the money to get exercises tailor-made to my body from a real physical therapist; I think it'd be really helpful.
I read a lot of stuff online about healing from top surgery before I got it, but I don't remember anyone really mentioning anything about the importance of physical therapy-type exercises. For me it's made a huge difference, physically and psychologically, so I strongly recommend talking to your doctor(s) about it.