Is it permissible for a woman observing her iddah to perform Umrah when she permanently departs the Kingdom?

Your mother must remain in your deceased father's house and not leave it except for a valid excuse. If she is compelled to leave it for her home country, there is no blame upon her. However, it is not permissible for her to travel to Mecca for Umrah; she is only permitted to travel out of necessity.