As is usual with accidents, it seems a number of failings conspired to result in the fatality.
From what I read, there had been a number of opportunities to inspect the release handle and no one had spotted the error with the safety pin.
It strikes me as a design flaw if a safety pin can be inserted with the handle in an unsafe position.
For the parachute to then fail to open was the final straw.
The manufacturers did have concerns over some aspects of the design and issued some safety bulletins, but not to all their users including this one.
Unlike morddwyd I have no knowledge of the devices and only paraphrase what I have read in the past from the coroners inquest.