What is the sign of purity from menstruation: is it complete dryness, or is seeing turbid discharge (kudrah) and yellowish discharge (ṣufrah) after dryness considered a continuation of menstruation, especially if the menstrual period does not have a specific number of days?

What you mentioned about the ruling concerning yellowish and brownish discharges is correct; it is the relied-upon madhhab of the Hanbalis. Therefore, if you observe purity by dryness or the white discharge, you must perform ghusl, whether you see white discharges or not. Dryness is confirmed by inserting cotton or similar material, which then comes out clean. As for the variation in the days of your period, whatever has occurred three times becomes your regular custom. Thus, a reduction in the number of days of your period in some months does not affect it.