Symptoms: dark loose stools, difficult/incomplete evacuation, severe abdominal distention from trapped gas, mucous. Diagnosed IBS. I think Proctitis?

Typical IBS symptoms. As you suspect, these symptoms indeed are consistent with irritable bowel syndrome. But they aren't particularly suggestive of proctitis (inflmmation of the rectum). Inflammatory bowel disease (e.g. Crohn's disease) can cause similar symptoms as IBS, and also can cause proctitis. So I suggest you raise these questions with the doctor who diagnosed IBS then follow her advice. Good luck!