As for a shouldered bolt, there might have been good reasons, such as the reduced diameter threaded portion if the bolt was shouldered not being of sufficient strength so as to not fail in the case of an ejection, and the main shank of the bolt not having sufficient clearance in the shackle for its diameter to be increased.
Umm, possibly, but there must be a better design! If not, why not use an appropriate 'feeler gauge' type tool to place between the inside of the shackle and the scissor linkage to tighten the nut against that is removed once the nut has been correctly torqued.
Having spent many hours sat on both Mk3 and Mk 4 seats, the face-screen pin was always inserted 'blind'. Quite a test of dexterity, flexible limbs and a long reach! I recall in the JP when dual we did each other's pin.
The seat pin was also fitted blind, but then both pins visually checked upon leaving the aircraft.