Congratulations! I've been wondering how things are progressing with your bundle-to-be, so thanks for the update. :)
From your list, I best like
Greta,
Meg,
Margo and
Maisie as "recognized" nicknames. I also really like
Mae. While not "intuitive" as a nickname for
Margaret,
Mae strikes me as very appropriate in "feel".
The only other ideas coming to me are
Pearl,
Ettie and "M".