But I posit that Our Harry will either take the slow path or the unstuck path to fix the timeline between Grave Peril and Mirror Mirror.
They used a very particular girl not just any innocent. Lydia.And, she had Casandra's Tears so even she could receive warnings about the future.To give one hint.There are two "There You Are" in Grave Peril both at the time of Bianca's masquerade.One is when Harry tells Bianca 'there you are.'The other is when Thomas tells Harry 'there you are.'I find it perfectly possible that the other side cheated during this book, by using time travel.And, Harry later is sent back to balance the equation. And part of his trip into the past is already written in previous book. It just needs to be expanded upon.Also, there is one WAYBACK MACHINE in SG that wasn't used. It was also primed to go back to FM/GP.
One other tidbit that I will offer as evidence of a return to FM/GP. We know that a choice Harry makes in GP is pivotal to events that occur in Mirror Mirror. Perhaps our Harry will realize the significance of that choice and choose to correct it, within that reality. Though he would have to do so in an indirect manner. But, it would still have him traveling back in time to events then.
I have fully laid out my theory by now and linked to it.But I posit that Our Harry will either take the slow path or the unstuck path to fix the timeline between Grave Peril and Mirror Mirror. This would be the speed chess portion of the plot.