Regarding mirrors, a work colleague, who commuted regularly used a helmet mirror and praised it a lot.
I used a handlebar mirror following a broken collar bone for a few months, when I had difficulty turning my head. I found it a useful indicator when NOT to move out, but could not rely on it to know when it was safe to move out. Found myself stopping on the left and looking back fully until it was safe to venture out and sometimes walking across. After a while, mobility returned to my shoulder/neck and I discarded the mirror.