Do clean clothes become impure when mixed with clothes whose impurity is doubtful, and can clothes whose impurity is doubtful be considered clean to get rid of obsessive doubts?

We advise you to turn away from the whispers (waswas) of things becoming impure. The default state of things is purity, and do not pay attention to the waswas unless you are absolutely certain that something has become impure. As long as you are not certain about the impurity of the clothes you mentioned, all of them are pure. You can refer to previous fatwas regarding the treatment of waswas and the minor, excusable impurities.