Invading a country with high crown laws

Solution 1:

I believe it doesn't work that way. The title can't pass outside the realm though inheritance. But after you gain the duchy, the title will be outside of the realm, so it can stay there, even if you die.

Solution 2:

Bit late perhaps but the good of it is that there are several ways to lose titles. Your way is through conquering the title of one of your neighbours. A duchy or a county which you claim by right of power - Your army beats his. That way you FORCE the title to pass outside of the High Crown Authority realm, and add it to your own.

High/Max crown authority only prevents titles from being lost through -inheritence-. Say a countess has a title and she marries someone (not matrilineally) outside of your Kingdom. (Countess of Norfolk marries the King of France for example). When she has children, they would inherit the King's title, and become King of France, but they would also get her title (Norfolk) and that would become a part of France: because as King of France the heir could hold it in his own right. High crown authority prevents this. When the Countess in this scenario dies, her title wouldn't go to the children of the King of France, but simply revert back to the King of England instead.