Is it obligatory to re-perform the prayers that were offered during the six-month period of istihada (non-menstrual vaginal bleeding) while having doubt about performing ghusl (ritual bath), and how is their number to be determined? And is the prayer of the mustahada valid if she only uses a sanitary pad without stuffing with cotton?

The basic principle is that doubt concerning an act of worship after completing it does not affect its validity. If the questioner is certain that she did not perform ghusl after the end of menstruation due to ignorance of the ruling, there is a difference of opinion regarding the obligation to repeat the prayers. If she is afflicted with waswasah (whispers/obsessive doubts), there is no harm for her in adopting the easier opinion. If she fell short in the duty of a mustahadah (woman with prolonged vaginal bleeding outside of her regular menstrual cycle) due to ignorance, she is not required to repeat the prayers, because avoiding impurities is only obligatory with knowledge and ability, and her prayers are judged to be valid. She must do what is required in the future.