The daughters of a duke are born with the precedence of a marchioness. The younger sons of dukes rank only as earls.
Similarly, the younger sons of a marquess are born with the precedence of a viscount, but daughters are born with that of a countess.
The younger sons of an earl are born with the precedence of a baron, but daughters are born with that of a viscountess and so on and so forth.
This is why an earl’s daughters are styled as Lady, while their brothers, except the eldest son, are styled as the Honorable.
The eldest son of course outranks both in each step of the peerage in fact they are accorded precedence one step down from their own parentage.
Eldest sons of duke ranks as a marquess, eldest son of a marquess as an earl, eldest son of an earl as a viscount, eldest son of a viscount as a baron and the eldest son of a baron above that of knights (thus the Hon ABC son of the Baron XYZ ranks higher than a knight of the garter!).
Wikipedia's list on precedence for England and Wales will illustrate how women are ranked one after another:
https://en.wikipedia.org/wiki/Order_of_precedence_in_England_and_Wales#Ladies
The probable reason Valentine Heywood gives in his British Titles is that sons succeeded to land in seniority provided there was no issue from elder brothers, but since daughters’ claims were co-equal they all ranked as heirs and thus daughters were accorded the precedence of an eldest son.
Suppose the Duke of London has 3 daughters, viz. Lady A, Lady B and Lady C.
Lady A marries the Earl of X.
Lady B marries the Viscount Y.
Lady C marries a commoner named John Smith who has no peerage.
Out of these three women, Lady C ranks as the highest after marriage since her precedence is higher than her husband's and retains her. The other two actually drop down in precedence since the women now derive precedence from their husbands. Similarly if Lady C remained unmarried while her two sisters married the two aforementioned peers, she would still outrank them.
To answer your other questions, the precedence the lady derives from her father is retained even during marriage, depending on who she married, she either elevates or is demoted as illsutrated above.
If Lady A who married the Earl of X divorced her husband, she can no longer have the precedence that comes with a countess and would thus revert back to that as the daughter of a duke, their birthright precedence does not change (as far as I am aware).
The excerpt from Heywood will probably answer all your questions.
S.S.