Do I have to make up missed prayers due to Istihada, the rulings of which I was unaware of, especially since I don't remember the exact number of days or how to make them up?

A woman is not considered to be in a state of istihadah (prolonged vaginal bleeding) unless the bleeding exceeds fifteen days. If a woman ascertains that she was indeed in a state of istihadah, there is a difference of opinion regarding the obligation to make up the prayers she missed, thinking she was menstruating. If a woman doubts whether she was in a state of istihadah, the default is that she was not, and she must disregard such whispers (waswasa) and not pay attention to them.