The commentary by Webber and Alexander makes some crucial points for the position of the "other woman" (Alex). But in attempting to rationalize Alex's behavior, they miss one vital aspect.
Webber and Alexander emphasize that Alex's insanity increases as she is continually shut out by Dan. This is the woman who slit both wrists when he was leaving her house after their weekend. Then she begged him for one last kiss, and proceeded to rub her blood all over his face.
I find it hard to believe that she was ever "quite sane and reasonable." From the beginning, this woman was not playing with a full deck.
JULIE M. DUNN