I inadvertently left the zone in between talking to Watcher Ibu and killing the zephyr, result being that he was dead by the time I got back to the tower. Also, I drank from the cauldron downstairs in that tower, thinking it might be one of those vitality-restoring drinks, but a couple minutes later realized I'd been warned repeatedly not to drink anything in The Corruption.
I have another save file from an hour or so previously, and am considering reloading from there to fix those mistakes. I was wondering if anyone who's played later in the game could give me a yes/no answer about whether those two things will make enough of a difference to bother redoing that much game.