I honestly don't see how the theory of that scene being a dream would hold up.
He gives Amin the pills in that scene - and those exact pills in the very next scene is what leads to the "assasination" of Garrigan. He clearly gave them the pills. The film is too realistic and straightforward to throw in a dream scene. I think that's kind of crazy.
And no, I don't think she had an affair to get out of her marriage. If that were the case, she would have done it a LONG time ago and/or wouldn't have been so upset about being pregnant.
"If you are going to do something, do it well. And leave something witchy."
-Charlie Manson