[Gnomasz]

I haven't played yet, but just reading the adventure I feel like it relies too much on delvers' curiosity, or rather odious personal habit of thoroughly checking every wall they encounter. The characters aren't presented with any suggestion that Merle might have been hiding something -- quite the opposite, Lee is described as very stingy on information about her family, and unwilling to pay for anything other than getting rid of the noises in the cellar. So gather the rats, cover the hole in the cellar's wall and the quest is complete.
I can see my players maybe checking the pile of dirt in the cellar and finding the tunnel, but to look further, they'd have to rely on the meta-game knowledge of "there's something on the map".
Which is a shame, because from the flow of information and style of writing it's the best adventure I've ever read -- pretty much the only one that doesn't feel like a boring manual I have to read, but rather something I might want to read.