Deliverance
I don't think it matters how the seat was activated. The fact is that once the sequence was initiated the system did not work as it should have. It would be the same as not fitting parachutes, useless.
As for the handle being in an unsafe state on 19 occasions, that's another issue entirely IMHO.
There is good reason to treat the seat firing and the shackle as entirely separate issues, and with respect, I don't think you've grasped it.
If Flt Lt Cunningham had been in the Line Hut, (after the penultimate flight) signing in the aircraft (certifying by his signature that among other things, the seat was safe) and a Red Arrows Engineer, maybe doing the Friday night refuel and tow to the hangar - the seat fires! The shackle has no bearing on the engineers death as he was not strapped in. It is certainly something to be investigated, but it is irrelevant to the engineers death.
War story - In the early 80's, a Canberra electrician got muddled up and fired the canopy detonators in the hangar. This was the focus of the Unit Inquiry. The fact that 50% of the canopy dets didn't fire was very alarming and the subject of a separate investigation altogether.