Does a small, involuntary amount of urine, released after urinating and cleaning oneself, invalidate the prayers, especially for someone afflicted with waswas (obsessive doubts)?

If you suffer from waswasa (obsessive whispering/doubts), turn away from it and do not pay attention to it. The basic principle is that nothing has exited from urine unless you are certain it has exited. If you are absolutely certain that urine has exited, then purify yourself, perform istinja’ (washing after urinating/defecating), and repeat your ablution (wudu’). If you doubt the time of the urine’s exit, then attribute it to the closest possible time it could have exited. As long as you are not certain that this happened in the past, your prayers are presumed to be valid.