IFMU, i used to know a guy in Oxfordshire who was one of the few in the UK qualified to repair wooden gliders. Since he also did composites and aluminium, interesting guy to talk to. I gather that spruce is only considered aero grade from so far above the ground to so far below the first branch!
How the Mosquito was manufactured is anyones guess, considering how many were made. I imagine that they graded wood for suitability in different parts of the airframe - clever chap DeHavilland.
Mart