I've also never been to La Sombra. One of the nice things about Moon Handbook Nicaragua is that, if it's in the book, we've been there and if we haven't been there, it's not in the book. Too many guidebook authors try to write confidently about places they haven't been and it usually shows.
I've got Nicaraguan friends and family in La Dalia, so I know the area well, and it's very enticing. So much of Nicaragua struggles with deforestation and this is still a nicely forested stretch of Matgalpa. Supporting businesses that rely on the environment will help keep it that way. Interestingly, you won't be far away from the folks that run
www.contracafe.com, and La Dalia itself is home to one of the famous Contra resettlement camps from the 1990s, land given to Contras who gave up their weapons in exchange for a house and a piece of land to farm. It's going to be a neat trip; please post your impressions when you return!