What is the ruling on street water when there is doubt about its purity? And does the legal pardon apply to it if it mixes with a small amount of impurity or when impure wheels pass over it? And what does the phrase "the impurity itself is present" mean?

Giving in to whisperings (waswas) is dangerous, and delving into assumptions only brings more of them. To get rid of this, the mud and water of the streets should be considered pure, and no judgment of impurity should be made against them. Even if you are certain of impurity, a small amount of it is excused by many scholars. The Maliki school excuses impurity mixed with mud and rainwater as long as it does not predominate, and if there is doubt about its predominance, the default is that it does not predominate. There is also an opinion that it is excused even if it predominates, which is what the apparent meaning of the Mudawwanah indicated, although some consider it weak. Ignoring whisperings is the best way to treat them.