Is the recurring sensation of urine drops descending after ablution, and the consequent doubt about the purity of clothes and Wudu (ablution), actual سلس البول (urinary incontinence) or merely Waswas (obsessive whispering)?

You have done well by ignoring the whispers of urine discharge and continuing with your ablution and prayer, for certainty is not removed by doubt. Continue to disregard the whispers and do not dwell on them. The default is that a person is free from incontinence until it is proven that he is afflicted by it.