Is the increase in the number of menstrual days—after the insertion of an IUD—during which bleeding stops and then returns, considered istihadah (non-menstrual vaginal bleeding), and is prayer and fasting obligatory on the basis that menstruation is only 6 days, and what about sexual intercourse in this case?

Every blood a woman sees during a period that can be considered menstruation is considered menstruation. So, if she sees purity (cessation of bleeding), she must perform ghusl (ritual bath) and observe her acts of worship, and her husband is permitted to have intercourse with her. If the blood returns during a period when menstruation is probable, she is considered menstruating and must refrain from prayer, fasting, and intercourse until the blood stops and she performs ghusl. The purity between two bleeding episodes is a valid purity, and there is no need to make up for the acts of worship performed during it. Yellowish and brownish discharges after purity are not considered menstruation unless they occur during the usual period or are continuous with blood.