Do you have a save before entering the summoning chamber? There is an auto-save just before that room, if you didn't keep going and hit another auto-save point. If you reload you would be able to return to Broken Valley to complete quests or tend to any other unfinished business.
However, once you enter the chamber I don't recall anyone reporting a way to bypass the summoning. Even if you could, there is a cutscene that gets triggered after Laiken dies, which may be the actual point the game poisons the Broken Valley / Sentinel Island areas, or it may not get triggered if you do manage to bypass the summoning.