What if Dan married Alex?

Oh, I don't think much would change. She was manipulative, angry, and self-pitying enough that she would probably drive them away not long after the birth. It seems literally impossible to think she could have a stable marriage for some time.

She would have pulled the Lorena Bobbitt on him eventually.

She was psycho, like the chick in "Play Misty for Me". Both of them would have been doting wives for a short time, then found something to go dark and apeshit over. Psycho women are not good relationship material.