Q: What happens when there is to power to be fed back, but there is no grid available?

A: The solar charger will charge the batteries until the absorption voltage is reached, and then reduces it's output. It switches to regulation on battery voltage instead of MPPT or output current. In other words: the batteries will not be overcharged.

Q: What happens when the battery is full and feeding back the grid is has not been enabled in the configuration?

A: See previous answer.