back on OOT:
there have allways been lots of theories as to why Margaret got such a free pass from the unseelie, and why Lea owes her such a major favor, amny of theminvolving complicated schemes as to lea is Harry's grandmother/ great gran , aunt's cousin..etc...most of which JB has blown out of the water at one time or the other.
(currently, there is some specualtion that Lea is Maclolm's mother, etc..)
but if Molly is Mab..none of that is needed.
she and Lea would have kept an eye on Maggie anyway, just as the mother of Harry and Thomas.