Thing is how could Astryn be so stupid?
Like one thing is supporting your friend and sharing his goals but giving away your birthright to him possibly breaking the continuous reign of an ancient dynasty?
It is not difficult to imagine if she alienated a huge chunk of the nobility with that move and are only going along Erwan’s campaign just because they hate Rhivenians historically to the core and would declare war on them no matter the reason,that Erwan is a bastard but on Pagonia they just not care much about birth and status possibly sealed the deal.
The men at arms and nobles would have followed Astryn with far more loyalty than they would Erwan,Astryn being Queen and Erwan being her retainer during the time of the story would make sense without losing the shock effect(that’s what basically Erwan as King of Pagonia is).
Astryn’s parents must be so pissed in their graves,her daughter just gifted her family’s power like it was a candy,going from Pagonians heir to incompetent low key assasin,then she becomes so obsessed with revenge that presents herself on their sworn enemy’s castle and just decides that openly plotting will get her somewhere.