Is money earned from selling bathroom hygiene products (for bathrooms that are believed to be unlawful to enter) considered unlawful or does it contain an element of doubt?

The cure for obsessive doubts (waswasah) is to completely disregard them and not to follow their jurisprudential ramifications. As for selling hygiene supplies for bathrooms, this does not fall under prohibited assistance in sin. This is because building, selling, and buying a bathroom, even if it involves exposure of 'awrah (private parts), is considered makruh (definition) tanzihi (reprehensible, but not strictly forbidden) by the majority of scholars, not haram (definition) (forbidden), due to people's need for it. Consequently, selling hygiene supplies to public establishments is permissible.