Is it obligatory to perform ghusl upon seeing white discharge after the cessation of yellowish discharge? Is sticky yellowish discharge considered yellowish discharge? And when is ghusl considered valid?

Your duty was to perform ghusl immediately upon seeing purity on the eighth day. What you see after that, in terms of yellowish or brownish discharge, which is not connected to menstruation, is not considered menstruation. Purity is recognized by dryness or by the white discharge (al-Qassah al-Bayda'). You must perform ghusl as soon as you see purity by one of these two signs, and do not pay attention to any yellowish or brownish discharge thereafter.