Our royal families have succession rules - why can't the Whamps? If it had been Elizabeth that died, Phillip would not become King of England, Charles would. Plus, the position of Head of House Raith is likely the one who can take it and hold it more than merely blood. Same for House Raith running the White court. Whatever House could hold it has it. Question is, though - if Harry suddenly finds himself a widower - does Mab marry him off to another Whamp? I am assuming that would come before the almost-infertile Lara has a child. But any would be coups... Harry is under order of the Winter Queen, doing his assigned duties. Attacking him unprovoked might be seen at attacking Winter.
Allying with the White Court per Mab's wishes will likely mean NO TAMPERING WITH THEIR INTERNALS without Mab's explicit permission, Molly. the actions of the Lady against an ally betrothed to the Knight by order of the Queen may be seen as treason in the Winter Court.
Molly can justify the other women. Elaine was before even Michael knew Harry. You aim at a 40-ish man, you do not freak if he talks to his high school girlfriend once or twice a year when there is no indication they ever kissed since he thought he accidentally burned her to death. Susan? She was still a kid, realizes now she was too young, and, Susan is dead. If Harry does not obsess over her, and he does not appear to, it's OK. Left out Luccio, but that was both while she was occupied as Lady and forced by magic, even if they did not know at the time. Murphy? Long time friend, Molly at the time was unavailable, being Winter Lady, and she is also dead. (being Harry's gf is a hazard to your health).
If she was not the Lady, and Harry not the Knight... she might make a move. I don't think she would just stand back. Leads to another question - I assume Winter cannot marry a mortal - but the Knight is not really Winter, he is their liaison.. maybe he would have been allowed a wife? But I think she has to lose the mantle before she and Harry could truly be together. (pact sealing sex a la Mab not included)