in his fevered state, he gets visitations from the lovely Andrea, the woman he slept with before he met Megan, when he was married to Betty

Not so fast, my friend... A friend pointed out to me that, she mentioned hooking up with him after seeing an opera at Lincoln Center. The first public performance at the Metropolitan Opera House was in April 1966. So either, they have a period detail wrong, or Don has already cheated on Megan.