have been told i could have had my parents and brother in prison and killed people and i am starting to wonder if i should find another job tbh.
I doubt that information came form a reliable Legally qualified source, It was from the electrician I suspect.
As second fix and powering up took place after the wall was plastered and the nail had been put through the cable and only hours before the incident, the fault should have been found by the Electrician in his final testing.
The purpose of testing at second fix is to check there has been no damage to cables.
It doesn't matter who put the nail in the cable or if it was cut by a plasterers trowel or nailed though the skirting by a Joiner, The Electrician should have found it.
If the nail was put in after second fix, that would be a different matter.