Do drops of fluid after urinating or passing wind, accompanied by a feeling of not being able to control urine after delaying and holding it, count as سلس ( سلس البول)? And how are undergarments purified from it? And is it obligatory to repeat a prayer after performing it in clothes that were soiled and then washed, but not changed?

If you ascertain the wetness, you must purify yourself from it by washing only the affected area. There is no significance to doubt regarding the nature of what exited as long as the doubt wavers between madhy (pre-seminal fluid) and urine, for both are the same in ruling. What you experience is not considered salas (incontinence) that necessitates a concession, because salas is the continuous discharge of the impurity throughout all the time. As for the faeces stain, it is sufficient to purify its location by washing, and there is no need to change the garment.