This is the only novel not covered on the site. While I surmise this is due to some of the potential continuity errors in it, I feel that at least a discussion of the book would be a good idea in order to see what fits, what doesn't fit, and any other thoughts on it so that an article could be created.
The book tries to wrap up multiple story lines around the Witch World universe. I think it really ought to have been made into more than one book given the amount of action taking place. In any case the loss of the Magestone/Mage Key is a very odd thing. There's no indication in The Magestone story that there's a power backlash from the stone leaving their reality.
The book seems to take place a year after Songsmith: "They had all gotten over the shock Trevor had provided last year… For, having been broken, that power…released time as well as captives, and in the weeks following his birth Trevor had caught up bodily and mentally with the years he had been denied" (p. 13). This is a problem since some characters (like Keris) should not be old enough to be riding about and fighting. If it truly does take place when Songsmith is supposed to be set (30 years after the Turning) then this makes more sense, especially given that Eleeri shows up later (and her story was also set 30 years after the Turning). Unfortunately there is no way Songsmith can take place 30 years after the Turning and still be consistent with the ages and events of other books. At best Songsmith takes place seven years after the Turning.
The story itself could take place in the same year as the events of "Falcon Hope" since it's mentioned that "lately also that invasion overseas made by strange seagoing race of fanatics whom only the skill, blood, and courage of Falconers and Dalesmen together had stopped" (p. 10). There's nothing inconsistent about him knowing this since the invasion by the Sultanites occurred in late winter and early spring - word of what happened probably spread quickly. "Falcon Hope", however, is definitely placed eight years post-turning. Even granting another full year, the story could not happen more than nine years after the Turning.
It would be easy enough to chalk up the 30-year-into-the-future as bad editing, but if it's meant literally there's no way Songsmith can happen that far into the future nor can Eleeri be around for the story if it's sooner.