to get jasper's bonus stats and epilogue, you need to get Katya's legendary ending, ,meaning you have to restore peace in Revaire without a civil war and all NATIONS must have the best peace agreements.
The fact that you only get the perfect ending only for Jiyel and Skalt, tells me that for the other negotiations that you lack either positive relationships with the others involved or you don't have the right amount of skills.
Choosing the right answers/ options during negotiations is the right step, but depending on the skill, which backs up whatever claim you choose your MC to say, it might backfire.
You didn't mention that you gained Avalie as an ally. You need her to get Katya's legendary ending. Without her you can't.
The Blackmailing is not relevant, you don't need to know Lisle or Zarad's secrets to get the legendary ending. You need to know Avalie's secret though, so that you are able to become her ally.