What is the legal ruling regarding a wife who wishes to complete her employment period to secure her right to a retirement pension, knowing that this requires her to work in a mixed-gender environment, and her husband rejects this, as he sees it as a violation of women's work and their mixing with men?

The summary of what you have mentioned is: If she completes the term in a permissible manner, there is no harm in that. Her teaching males—if they are not yet بالغين (of age/puberty)—is permissible. However, if completing the term necessitates falling into religiously forbidden acts, then she must avoid it. Her waiting until retirement age is a way out and a solution.