Does the fast of the fifth day of the menstrual cycle become void if the girl fasts it without performing ghusl, or if she fasts it and performs ghusl, then sees blood after Isha? And must she make up for this day, especially since her menstrual period ends on the fifth day?

There is no basis for the statement that ritual purity is not complete until after the seventh day, or that fasting is not valid without performing ghusl. Ritual purity is not a condition for the validity of fasting. So, if a woman becomes pure before the dawn of the fifth day, she is obligated to fast, and her fast is valid. She must perform ghusl to perform the prayer at its appointed time. Delaying prayer beyond its time is a grave sin. If she becomes pure and fasts, then sees blood after the 'Isha prayer, her fast is valid. However, if she becomes pure during the day on the fifth day, the fast of that day is not valid, and she must make it up. Menstrual cycles differ from one woman to another, and each woman acts according to her own cycle. Whoever becomes pure before the end of her usual cycle should act according to the apparent purity.