To whom should I address a complaint regarding a physician's treatment during an emergency room visit with my mother?

I would start with the head of the ER department - if that particular doctor happens to BE the head of the ER department, then I would move on to the hospital administrator.

I've had that experience as well, and it was unpleasant. We're there to help the same person - our loved one.  The doctor needs to understand that you're not the enemy, you're advocating for your loved one, and in some cases, you have to be their voice and memory, or nothing will get accomplished.