I can't say that Harry's wife and Dahlia are the same person because one is real and the other is a part of Cheryl's imagination. We can assume Harry's real wife is Dahlia because of the scene where the older Dahlia says she's his wife. She definitely represents Cheryl's mother, she just might have a bad opinion of her and thus she's a slut/drunk/whatever game you're playing
She may have had a rough relationship with her mother as most teenagers do. Especially if Harry died in a car crash after breaking up with his wife and telling Cheryl he loved her (like we see in the love lost ending).
I think she painted her father up to be a "hero" while she blamed her mother for what happened. She thought of her mother in a negative way (Dahlia) and imagined a perfect mother she wished she had (Michelle). As progress is being made with Cheryl, John (Harry) breaks up with Michelle, and she begins to feel sorry for her mother. Even if she is still in denial about her fantasy mother not being real.
And MariaRising, thank you.