This does not clarify the parents of LADY FRANCES SOMERSET.
Which Mary are you talking about who is the 2nd issue?
Neither of these women named Mary are ancestors of Queen Victoria which seems to be your claim to the nullification of her presumption to the British throne. Queen Victoria was the daughter of Edward of Kent, the next heir as he was the fourth son. All of his heirs would precede any of his siblings heirs. The next would be Ernest, Duke of Cumberland (the fifth son) and all of his heirs. They are before Adolphus, Duke of Cambbridge (the 7th son) and any of his heirs.