On the MB 'no design fault' notice, you need to read it with lawyerly attention to detail (my bold):
satisfied that neither a mechanical nor a design fault were to blame for the fatality
They are not denying that a design fault (eg in the handle or safety pin) might have caused the ejection. However, since the ejection should have been survivable, the
fatality was due to the failure of the parachute to deploy. As soon as the over-tightened bolt was discovered, therefore, MB could issue that statement to reassure their customers without needing to establish why the ejection had happened in the first place.
Now, one for the accident investigators among you. A service inquiry is required for major injuries. If Sean's parachute had deployed, it is still quite possible that he would have broken a few bones and precipitated an SI. The cause of that accident would be the pulling of the seat handle, with its unsafe condition as a contributory factor. Given that the outcome was worse, ie fatal, does that make the over-tightening of the shackle bolt an aggravating factor to the same root causes? Or is the over-tightening of the shackle bolt now the cause of the accident, with the unsafe condition of the handle and its pulling as contributory factors? And would the distinction have any significance for liability or culpability?