All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
It seems, dear Brother, that you are suffering from some sort of obsessive whisperings (waswaas). We advise you to totally disregard such whisperings; this is the most effective remedy for them.
As for your first question, if you suspect that your clothes are stained with impurity after having used the toilet or at any other time, then you should not do anything because the basic principle is that your clothes are pure. This does not change until proven otherwise with certainty. As long as you do not know with certainty that your clothes carry impurity, you are not required to purify them. Please refer to fatwa 206935.
As for your other questions, please refer to the following fataawa 82085, 85661, 85203, and 92050.
Allaah knows best.