My guesses about these relationships?
DAHLIA = she is apparently asexual... it's not an important issue for her and she sees MC as her savior, she idolizes him... she sees him as that little boy who "saved her", so she has a maternal love. Nothing will happen;
MARGARET = she lives with guilt. clearly. regardless of her age, her head is only about what her choices caused in MC's life, MC's family, her own daughter and granddaughter. Nothing will happen;
IRIS = something close to what I said about Dahlia, but I believe that Iris is more open-minded and there is a greater chance of something happening between her and the MC;
MILLIE = until then, she was the woman of the "traditional family", with a puritanical view, that a marriage is forever, maybe even with the idea of "sex only after marriage" and things like that... but with everything she saw with her daughters, MC, how he didn't confront her, didn't raise his voice, didn't treat her badly (he just defended his girlfriends, who happen to be her daughters),... clearly her view has changed. She was even high on weed and getting several girls drunk who weren't old enough to drink. This, to me, is a clear sign of someone who is trying to deal with confusing feelings. She was cheated on, her marriage is over, she feels left out, wants to be loved and, with her daughter encouraging her, it's only a matter of time before MC wants to take care of her too.