safety instructions are a broad brush
to try to cover every eventuality, and as PaxBoy points out so well, the application of the safety instructions are best locally (ie in that seat) interpreted.
For example last SLF sector, my seat would not stay in the upright position for take off or landing, the same for another seat close to me. this was know by the CC prior to take off. using the 100% rule the flight should have returned to the gate, and either the seat repaired, of the 2 pax off loaded.......
On a recent sector, there was no life jacket under my seat, it was missing....when the CC advised by me prior to push back, no action was taken to rectify this breach of safety regulations (again full flight).
In this part of the world, push back, with passengers still standing, and trying to organise their family group, is the rule rather than the exception.
So the rules are there to be observed, when practical and possible...
please do not make me turn off my internal electrics, the mess would be unacceptable...