Legal Question in Mediation in California

A settlement is a form of contract. Any offer of a contract must be accepted within whatever time-frame the offeror specifies. It is not duress to make an offer conditioned on it being accepted the same day. If the mediator and your attorney were telling you that you had to decide that day, it was almost certainly because your employer was saying the offer was only good for that day. Once you accept the offer, you have no right to rescind it unless legal grounds for rescission exist, such as fraud or true duress (which has to be something very serious, like they are holding your child hostage and will kill him if you don't accept; merely being "under pressure" is not duress).