What is the ruling on completing mixed-gender and exhausting university studies that are not beneficial? And is it obligatory to obey one's parents in completing them in order to marry a man who agrees to their completion?

There is no obedience to parents if their obedience leads to committing a religiously forbidden act, such as studying in mixed-gender schools. A woman living in her home and seeing her children is consistent with her natural disposition and safer for her faith. You should strive in supplication that Allah expands the hearts of your parents to the truth and that He marries you to someone whose religion and character are pleasing, and be keen on the means of attaining Allah's love.