I would add that Mab's resources; while impressive, are mostly at the Outer Gates. Most of the rest will be assigned to fighting the Fomor. Plus, Mab only puts herself in harm's way to strike at an enemy under the most dire circumstance. This wouldn't be it. That's one explanation. Another is the possibility that any attempt by the Outsiders to get the White Court on their side won't be aimed at the entire White Court, it will be aimed at recruiting their leader. Even if the demons which infests each White Court vampire was a product of something the Outsiders did, it doesn't follow that the White Court become the Outsiders natural ally. It may have been the manipulation of Lord Raith that makes it seem that way.
Lara has access to her father's library and any notes he may nave kept. Getting some guarantee of controlling a surviving principality and becoming immune to magic might be an enticement to switch sides if it looks like the Outsiders might win. Mab getting Harry next to Lara fits in with my earlier stated idea that the forced marriage is Mab's insurance policy. Keep your friends close, keep your enemies, or potential enemies, closer.