Chug, DV
The RTI was not around when I worked on Mk10 seats so I would not like to comment on that process.
Parachute Headbox changes were a fairly common event, either when called up for bay maintenance, or certainly when I worked on Hawks, contaminated with vomit!
To replace the Headbox Assembly it was a matter of disconnecting the Harness top and bottom locks, sticker straps, scissor shackle bolt and then 2 bolts to take the Headbox from the Seat structure.
Fitting was the reverse, but the new Headbox always came from the Para Bay with a brand new bolt and stiff nut. This is critical because if you re-use the stiff nut the plastic insert becomes less effective. If the RTI has been calling for that connection to be disturbed every 50 flight hrs, were they replacing the nut, or just reusing the old one?
With regard to tightening the Nut, as I have stated previously on this thread, I was taught 'max 1.5 threads showing above the nut,' but critically to ensure that there was freedom of movement of the Drogue Withdrawal Line in the Scissor Shackle. As for exactly how that was stipulated in the 1J I don't recall, but we were certainly aware of over tightening said bolt at Chivenor back in the early 90s.
For me, a major factor in this incident was leaving the seat on the actuator down limit because the same pilot generally flew the same aircraft. This was against the procedure for af, bf and tr and would seriously restrict the view of the Seat Pan Pin for anyone entering the cockpit. Had the Actuator failed (as they sometimes do) it makes it a much more difficult task to remove the seat for rectification.
In 6 years on a TWU on a Sqn of around 20 aircraft, student pilots, fast turnarounds, high pressure, I never saw a Seat Pan Firing Handle in this condition, although I was aware that it could exist.