Thanks for checking it out and leaving your thoughts!
Like everything in TTRPGs, it find that it very much depends on your table and your players - many goodhearted adventurers would want to save the villagers, as many as they can anyway, and the Mouth wants to see the ritual to completion more than it wants to kill newcomers (especially if it may enthrall them as well and speed up its work). If a party of adventurers flees at a sign of danger, that's certainly a choice they can make, but it's not one I think many characters worthy of the term "adventurer" are likely to choose (after all, why do most adventurery things, in that case?). Also, if the GM telegraphs the threat at the heart of the adventurer well them leaving could even have interested play down the line as a region-wide, potentially world-ending threat is now on the loose - all because they chose to overlook it.
That said, it's not in my style, for this kind of very small release, to feel like I need to trap the player characters inside or give a reason for adventurers to investigate - I assume they will because 1.) that kind of inquiry and action is often at the heart of the game being played and 2.) the GM at the table can easily craft far more tailored hooks for a character or campaign than I can (perhaps the Mind Masters are allied with another dark god the party has been investigating already, perhaps the village is home to a character's mother they were planning to visit while on their way to the next location, etc.).
Beyond that though, I like your slow play option - maybe even having the Mouth (in the guise of another NPC) offer the party an old building to use as their camp and slowly have things reveal themselves over the following fortnight. Same with adding more clues regarding the Mouth - there is one as written in the adventure but more couldn't hurt (and locating the Mouth or avoiding it will still prove difficult). If I were to expand this out further or add to the Mind Masteriverse as well, I'd definitely be thinking back on your critiques and seeing how I could address them further too!