i just got this... i did a run-through being as passive as possible and succeeding at every charisma and intuition check. If there was a fight after a conversation, i reloaded before it and changed my party to include whomever was associated with that group to avoid a fight if possible. i dont know if this all matters. I also elected Duncas as high king
The only difference between my last 2 playthroughs was
this leads me to believe the only things necessary to get this ending are the things spoken about in the ending, which are all associated to the different side quests. You may not need to get everyone to friendly as long as you finish all the side quests to the proper conclusions (proper as in best for everyone as a whole). This, at least for me, was achieved by bringing the people associated with the faction the quest was from, and always picking/succeeding the options for charisma and intuition. I dont know for sure if this is the concrete solution, but thats my conclusion about it from my experiences.