Is money earned from a woman's travel without a mahram—given the presence of safe companionship and prior arrangements—considered lawful (halal) or unlawful (haram)?

A woman is not permitted to travel without a mahram (definition), based on the Prophet Muhammad’s (peace be upon him) saying: “No man should be alone with a woman unless she has a mahram with her, and no woman should travel except with a mahram.” This is unless there is a necessity, which is assessed on a case-by-case basis. We do not see a necessity in your travel. You can benefit from the funds you received, provided that the work of the delegation was not unlawful.