I think mehr should be discussed between the girl and the guy i.e. if they have that comfort level and frankness between them. I have too often seen elders come in with their egos and create unnecessary tension regarding mehr. It should be a reasonable amount, not too high, not too low and ideally should be paid upfront after marriage. However a girl can choose to forego it if she wants.
But in case of a divorce, the amount has to be paid to the wife. In case of a Khula, the wife loses the right to that money if it hasn't been paid. If it has been paid, she might have to return it if the husband asks for it. If he doesn't request it, then she is under no obligation to pay it back.