I do not know whether the pilot was IFR or VFR, however in the past, the cray runs have been mainly VFR. Which brings the question of VFR into IMC or pushing a bad situation to stay visual.
I believe airservices need to remove the charge for IFR plans. There are so many operators that push their pilots to fly VFR when the wx is sh** to save $$$ (one big operator on Horn springs to mind)... In the past, there have been so many near midairs in the area with VFR pilots in cloud not being passed on as traffic.
As for the possibility of the wings clapping - these are old machines, and who knows what kind of stress they have endured over their lifetimes with many pilots replicating Bob Hoover manouvres. An old fatigued wing spar may not survive the inside of a thunderstorm.
Anyway my thoughts go out to the guys on Horn and those that knew the pilot. Good luck with the search.