IMO, on Mab's sudden alliance marriage:
1. Mab is scared of a breakdown between her court and the white court due to Thomas being removed from the picture. She is also concerned Murphy's death may affect her winter knight in ways that make him even harder to control, and incorrectly guesses any woman will do as a replacement (Fairy thinking).
2. She is POed about how the Council has treated her knight, who is *hers* and is likely lashing out at them also.
3. She's also up to something, and I think she doesn't really think this alliance will go through as a marriage, but might be trying to get Dresden and Molly to work out their unresolved feelings. I think she's trying to give Harry a bodyguard not only due to the council threat toward him, but due some upcoming events where a super-powered girlfriend would be able to keep up and survive.
4. In the short story int he back, it looks like Mab watched Disney's Frozen with Molly, so I think we know where the sudden marriage idea is coming from there.