Regarding alternate solutions: there was one, and I'm honestly surprised no one tried it, but it was definitely not preferable to the given solution.
The ending was a bit rushed, yeah. I wanted to establish something more interesting than "Veraius is bored because he 'won' so he's leaving," which was the initial idea and one I had some darker plot points for, but I really genuinely did not think it would come to the bad ending, especially because I started piling on the hints. On the final (real-world) day of the event, I had the riddle on the library counter, then removed everything else from the library counter and placed a pile of greenstalk opposite the letter, and also placed a greenstalk directly in front of the Cerridwen shrine in the Temple after some characters assumed prayer to Cerridwen was the solution (several critters went into the temple to pray but none prayed to Cerridwen or saw this). After neither of these were noticed I moved the greenstalk pile in the library so it was adjacent to the riddle paper. I almost screamed when I watched someone take the pile of greenstalks to eat.
And yes, it is admittedly poorly established that greenstalks are a garlic equivalent, but also no one ever said anything like "gee I wish we had some garlic for this vampire," so I don't think that it being more clearly telegraphed as garlic would have changed the outcome haha
Overall I'm very glad the event was well-received and that people had fun. I wasn't sure how a long-term GM antagonist would do with the formula of the game, nor did I expect him to be around for so long.