Is it permissible for a husband to prevent his wife from performing Umrah with colleagues—males and females—on the pretext of intermixing, and is it permissible for a woman to travel for Umrah without a mahram?

A woman is not permitted to perform a voluntary Umrah if she is not accompanied by a mahram, or if her husband has not given her permission, because the husband's right takes precedence over voluntary acts of worship. Furthermore, the husband has the right to prevent her from traveling if there is unlawful mixing.