Is my action correct in fasting and praying when blood reappears during Ramadan, given my inability to distinguish whether it is menstruation or istihada (post-menstrual bleeding), and does the divorce issued by my husband during that period take effect?

If the blood at the beginning of the month was menstruation, and you purified yourself on the sixth day, then the blood that returns on the seventeenth of Ramadan is not considered menstruation, because it exceeded fifteen days when combined with the previous blood, and the period of purity between them did not reach thirteen days. Your fasting and prayers during the time of Istihada (non-menstrual vaginal bleeding) are valid, and the divorce is effective in all cases.