Does the ghusl become invalid if tap water intentionally enters the urethral opening during the ritual bath for janabah, knowing that the person believes the water that enters the urethral opening to be impure?

You must ignore the waswas (whispering of Satan) and continue with the ghusl (ritual bath). For istinja' (cleansing after relieving oneself) with water, washing the outer part of the exit suffices, and it is not legislated to insert water inside, as that is an extremism in religion. If water enters inside the penis through dripping and then exits, it is najis (impure) and nullifies wudu' (ablution), but it does not invalidate the ghusl. Therefore, your ghusl is valid.