It’s just as Violet applies shields to herself and allies, the shields will stay as the skill states, and Violet’s death won’t make them disappear. Same for her Merida disk, it works for whole battle because it works on allies shields as well.
For Fear and Joy there isn’t any duration of buff, but it can be removed by removing their stacks.
If Fear is so big problem, upgrade Tron, Chef and Hamm.