Is it permissible for someone who went for 'Umrah to stay behind and not return in order to perform the obligation of Hajj, and which is more befitting: Hajj in this manner or marriage, if their times coincide?

Hajj without a permit is valid, but it is not permissible to expose oneself to danger and humiliation. Marriage takes precedence over Hajj for one who fears falling into sin (al-anat), but for one who does not fear falling into sin, Hajj takes precedence for them.