I may well have missed it despite careful perusal of all the posts, but what DID initiate the seat firing ? I have not yet seen the SI report.
I don't think it was conclusively determined, with the handle already pulled out of the holder it could have been anything from simply knocking it with his leg to putting something in a pocket, and sadly the only person who could answer that question is no longer with us, maybe the service board of enquiry will elaborate on how much movement was required to initiate ejection when the seat handle was in the raised position.