Does whatever comes out of the urethra after istinja (cleaning oneself after urinating or defecating), and is most likely assumed to be water from istinja, invalidate purity and necessitate its renewal, and what should someone who suffers from waswasah (obsessive doubts) do in this situation?

The cure for obsessive thoughts is to ignore them and pay them no mind. So, when you cleanse yourself after relieving nature, wash the area until you are reasonably certain of its purity. A strong presumption is sufficient; certainty is not a condition. After this, do not investigate or search to see if anything has come out of you, and do not rule that anything has come out unless you have absolute certainty, to the point where you could swear by it.