Canonically, your character didnt know that you were going to the Wall the fight Ilberd. You knew that the Griffin was running a ploy to trick the empire into thinking the alliance had attacked. You didnt even know if it was the man himself leading the attack, or his body double. So you went to find the person leading the attack, and convince them to withdraw and stop fighting. An offer to speak before the alliance leaders was likely to be your opening bid. When Ilberd took off the mask, it was likely not only too late to change the plan, but too late to change the outcome of the fight.
Lets say you were given the option and you put him down at the end of the fight without the long speech. His people are still sacrificed, the eyes are still there to lend aether, and his death still completes the summoning ritual. Shinryu still forms, only now its "your fault". Either Y'shtola or Papalymo would recognize the pieces of the ritual and give you the requisite lore dump. Then Papalymo still dies because we have to try to contain the newly forming primal.