I actually feel I have to explain the same to you for thinking that a written contract is worth so much more then a traditional oath spoken before the entire realm.
Breaking agreements such as say, the oaths they already swore to us? Do you not remember how the dance happens in canon? We already have everyone's oaths. Our half brother could just say that no agreement signed by Corlys rather then himself could possibly be binding upon him, and it'd make about as much as the figleafs which everyone who canonically used to ignore their oaths when they declared for Aegon used. They're going to have a custom they could point to in Andal succession laws to justify our brother inheriting before us no matter what, because that's the exiting custom oaths or not, to the point that
Viserys' master of laws went to his executioner proclaiming that a man must inherit before a woman. Getting those same oaths in writing rather then verbally isn't what would have changed his mind.