What is the ruling on very small drops, difficult to determine whether they are urine or prostatic fluid (madhy), that sometimes exit without sensation, or during istinja (ablution after defecation/urination), or a long time after a wet dream? Is it obligatory to repeat the prayer because of them, or to delay the prayer until they stop, or to delay it until after the Friday prayer?

There is no need to inspect or search. Act according to the default principle, which is that nothing has exited until you are certain otherwise. If you become absolutely certain that something has exited, then you must perform istinja' (ablution with water to clean oneself after defecating or urinating), re-perform wudu' (ablution), and repeat the prayer. There is a facilitation among the Malikis that istinja' is not obligatory for this discharge if it is frequent. If you find something and doubt whether it exited before or after the prayer, then rule that it exited after it. If the hadath (ritual impurity) is continuous such that it does not cease during the prayer time, then your ruling is that of one afflicted with sals (incontinence): perform wudu' after the time for prayer begins, and pray as much as you wish with that wudu' until that time expires. Do not delay the prayer until its time has passed.