I doubt they killed Icheb off because of what the actor did. They wanted to do something to effect Seven. Why not kill someone she saw as almost her own son? Better than creating a new character just to kill off.
Why should reoccurring characters be immune? That's silly.
Not immune to death. Just maybe don't go all gore on a character was in enough episodes to sort of be part of family. I know Voyer didn't have as many of those types like DS9 but they had a few. Ichep was one.