The Mushroomery though is actually a music track from an old FF game, the very first in fact - it's real title is simply Matoya's Cave from FFI, and composed by Nobuo Uematsu. But I agree it's a beautiful track (when I first entered the Hinterlands and checked the map and saw Matoya's Cave listed as a location on it I admit I cheered then). 
As for Y'shtola, given that we've already seen
three Sharlayan Archons meet their ends already (Louisoix, Moenbrdya and Papalymo (although the jury is still out on that last one), and not including Minfilia (who isn't an Archon nor Sharlayan), I wouldn't be surprised if Y'shtola meets a similar fate, as it seems that as Y'shtola, Thancred, Yda and Papalymo have been part of the story since the very beginning of version 1.0, it's possible this is deliberate, to clear the slate for a new cast of main characters to take over.
Or maybe I'm just reading too much into this.