If the time of the son’s death is known and the time of the father’s death is unknown, then in this case, the father inherits the son and not vice versa.
If the doubt is conventionally normal, and not a type of waswasah, one should pay attention to it, and therefore wash until he is certain that during the washings the water has reached all parts of the face or arm. If the doubt is a type of waswasah, then one should ignore it.
It is permissible to wear it except in prayers. It is considered to be Najis so one should purify whatever it touches with dampness, like if one touches it with wet hands.