Does an increase in menstrual days beyond the usual period constitute menses that prohibits prayer and fasting? And is the complete dryness of blood for 21 hours after the end of the usual period considered the cessation of menstruation, even if blood reappears later before the fifteenth day?

The preponderant opinion is that any blood a woman sees during the time menstruation is possible is considered menstrual blood. So, if a woman sees purity, then blood returns to her within 15 days (which is the maximum duration for menstruation), it is menstrual blood, even if it is not during her usual time. The purity that interrupts menstruation is a valid purity, so if she sees purity, she must perform ghusl and pray. If blood returns to her, she becomes a menstruating woman again and stops praying and doing what a menstruating woman refrains from. When she becomes pure, she performs ghusl. This entire period is considered menstruation as long as it does not exceed 15 days; if it exceeds that, she is considered to have Istihada (non-menstrual vaginal bleeding).