Is ghusl obligatory if a woman experiences a transient urge of desire along with the fear of fluid discharge, and after a few minutes finds vaginal wetness that she is accustomed to, but fears that something may have been discharged due to her pressing her legs together?

It appears that what exited is madhy, which is a thin, sticky, white fluid that is discharged upon arousal, not with lust or gushing, and one might not feel its exit. It occurs in both men and women. The discharge might also be vaginal moisture. If you are uncertain whether the discharge is maniy, madhy, or moisture, then ghusl (definition) is not obligatory upon you, and you have the choice to assign it the ruling of one of the things about which you are uncertain.