Is the drop found on the tip of the penis after bathing semen or urine, and did it get on the towel? And does the small trace of feces that emerges after complete purification contaminate the last water used for istinja'? And is the unknown drop that got on the liquid after istinja' from urine impure? And is the unknown wetness that a person finds impure, necessitating its washing?

The towel is pure because certainty is not removed by doubt. Semen does not necessitate ghusl unless it is discharged with desire and a gush. It is recommended to start with tissues then water for Istinja' (cleansing after defecation or urination), not the other way around. Checking the area after Istinja' is an exaggeration. The garment is pure. Water that separates from an impurity does not defile the garment if it does not touch it. If you doubt the impurity of a drop, it is pure. If you doubt whether wetness is semen or madhy (pre-ejaculatory fluid), you may choose one of them. The preponderant opinion is the purity of semen. For madhy, sprinkling water is sufficient, and a small amount of it is overlooked. What you mention in your questions indicates scrupulosity (waswasa), so you should consult a psychiatrist.