Silver spoons were used by the wealthy because they are pretty and the wealthy had minions who could keep them clean-- and also because there was a tradition that silver cutlery (in effect spoons as forks were not yet popular), turned black in the presence of poison.
Since silver has a propensity to turn black at the drop of a hat let alone poison is not news to anyone who has had to polish silver, so probably poison may have inadvertently been detected. However the silver tarnishes if a silver spoon is used to eat a boiled egg!
As for the 'silver spoon in the mouth' I think Bill explained that.
In plagues everyone was felled, but the wealthy used to escape to their country houses when the plague season happened, possibly taking their spoons with them, but I don't think there s evidence that they thought that the cutlery would protect them from the illness.
