You realy cant win in this type of situation, if you asked the passengers to stay in the gate area there would be complaints of lack of facilities ,being treated like cattle etc., but when you allow passengers the freedom of going back to the terminal , you run the risk of missing a slot if it is brought forward. It is nothing to do with professionalism, the agent was doing what she was probably told and was working on the information at the time. In my airline, we board , send a ready message and hope the slot somes forward, that is the procedure, but try getting some crew to follow this is not easy, they prefer to make their own rules.