I think giving titles to their sons was pretty common which then resulted in succession wars after the ruler's death...
Yep, I think matrilinear marriages for your daughters are a must with agnatic-cognatic succession. You just have to micromanage female family members who inherit titles. They are capable of marrying patrilinearilly (is that a word?) if you don't monitor them carefully.
Besides, I don't really understand why all dynasties default to agnatic-cognatic gavelkind. E.g. the House of Árpád had tons of (fairly bloody) succession wars with some supporting 'agnatic seniority' and some others 'agnatic primogeniture'. Any female on the throne would have caused all vassals to auto-revolt instead of giving a -10 to opinion
They just didn't have the right mindset for it