Does white discharge that turns yellowish when wiped take the ruling of white discharge indicating purity (al-qassah al-bayda’)? And does a drop of blood that appeared after 'Isha on the fifteenth day of the cycle count as menstruation requiring ghusl? And is the ghusl valid after seeing the yellowish-white fluid? And when is the end of the fifteenth day calculated?

You are afflicted with waswasah (whispering doubts), so ignore them. If you see white discharges, like the usual wetness, then you have purified yourself, and a change in their color on the tissue is of no consequence. The calculation of the fifteen days is based on the time when blood was first seen, and its equivalent after fifteen days marks the completion of the maximum duration of menstruation. If a woman sees blood at a time suitable for menstruation (within 15 days), it is considered menstruation, and she should perform ghusl (ritual bath) after its cessation. If the blood stops, you perform ghusl and pray. If a drop reappears within the 15-day period and you then see white discharges, you should promptly perform ghusl again, and pay no attention to the change in color of the discharges on the tissue, while striving against the waswasah.