In addition to abovementioned:
- get the quest, talk to both factions
- meanwhile finish ,,A Sinking Feeling" quest
- get to mediate the feud
- track Nezza and uncover the plot of robbing the Bardatto's vault
- tell Ezzali about it
- speak with Valera's and agree to help their robber
- when entering the Bardatto's Mansion I was attacked, then after wiping out all of the opposition and taking their loot I returned to Valera's
- Valera thanked for resolving the matter for them (Unique Sword as a reward) and agreed to negotiate with already killed Bardatto's
- In the cutscene Ezzali is alive again and the two factions settle an aggreement
- after the scene you get reward from Bardatto's (Bardatto's Hull) and again Unique Sword from Valera's
I don't know how to start negotiations without taking one side or another, but the outcome above is the most profittable methinks.