How can one deal with obsessive thoughts that cause doubt about disbelief, especially upon seeing foreigners and fearing the unwitting excommunication of a Muslim, while at the same time fearing not to excommunicate Jews, Christians, and polytheists?

The treatment for waswas (whispers/obsessive thoughts) is to turn away from them and pay them no mind. No matter what thoughts or delusions Satan casts into the heart, one should not care about them. If Satan suggests that this is a lack of concern for religion, this too is from his waswas, so one must turn away from it and ignore it. Giving in to waswas leads to great evil. One must firmly and definitively affirm that they are upon the religion of Islam. The occurrence of waswas is evidence of sound faith, and struggling against them is rewarded. Regarding the example mentioned, there is no need to judge a person whose religion is unknown as either Muslim or disbeliever. As long as a person considers every Jew and Christian to be a disbeliever, this is sufficient for him in the sight of Allah.