Deus ret. describes it quite well.

You can either have an agent (Princess) which can't be an heir or a king,
or you can have a female heir which will be in a military unit from the start and can't move on the campaign map freely without waging war.
Modding this female heir would mean
1. you always have all heirs labelled either "Princess" or "Prince" not regarding their intented gender. It's either or
2. when changing some portraits to female they'll be randomly assigned to male heirs, too
You could mod a female hero "Queen X" and also mod the title "King" to "" so that you don't have a "King Queen X" and can assign a portrait to her, but that's about it. This also results in Kings not being named "King X" but just "X".

To sum it up, the idea you describe is not possible.
Sorry.

R'as