@Elmithian : Except all that would have been tried the first time though, her grandparents did voice objections and they have very valid reasons for why they should never do what she knows they will do. Talking will not work when you're dealing with people who do not listen, and the first time her parents were not listening and it was not due to a lack of an emotional bond.
Her parents created the infighting that weakened their country because they did not care about anything other than the position going to the child who had 1 ability the other did not. It is tradition that is heavily ingrained in their family, for very valid reasons that they all knew about, and her parents knowingly broke it for trivial reasons.
The problem is not something that she can talk her way out of, this is 1 test result at 1 point in time that she can not modify causing multiple people to knowingly break their deeply ingrained rules and discard years of work and preparation. It is not something she can talk her way out of, especially the way parents treat their children's concerns. That and the 180 they made in the first iteration means that talking it out is to risky, not when a nation's security is as risk, the MC is not going for a plan that MIGHT (but is unlikely to) work, she is going for a plan that WILL work.
Talking has next to no chance of success, this is not a case of them not knowing what they are doing, and the parents did not listen when the grandparents (who are badasses that they respect) objected the first time.
The problem is only 1 person can be in charge, and only that person inherits the house, peerage and title.
IT CAN NOT BE F%*&ing SHARED. That is not how noble titles work, and their plans would have had had one being subordinate to the other by default.
Everything with this is just made worse by the noble politics involved. She knows she is dealing with people who will not listen and abandon everything for foolish reasons. the only thing she can do is minimize the scandal and damage it would cause to to the nation's security. Talking it out has to low a chance, the position can not be shared, so all that is left is for her to remove herself from the line of inheritance. Once she is out that basically means no longer being a noble as she can not stay a member of the family or else she would set a precedence where their tradition is shot and another crisis will just happen in a future generation.
As long as she is still in the nation (or even presumed to be alive) it causes problems. In disappearing she likely needs to fake her death as well, otherwise it could cause problems with her brother taking the position and the perceived legitimacy of him inheriting the title (and he needs to be seen as legitimate). Heck I would not even want to be in that nation afterwards, so surviving in another country kind of makes sense.
The only other option would be for her to marry someone whose status required her to be removed from inheriting the title, which is pretty much just the prince, since lower or equal rank would not fix the issue.