What is the ruling on remaining silent about Muslim girls traveling to study in non-Muslim countries without a mahram, and is this permissible if they initially came with a mahram who then departed?

Calling to Allah, enjoining good and forbidding evil, and offering sincere advice are obligations upon every Muslim, according to their capacity and with wisdom. If minimizing discussion of non-essentials encourages people towards religion, then that is the correct approach. There is no threat of concealing knowledge if refraining from clarifying some matters is out of fear of alienating people or them not listening. However, mere fear of not being listened to does not preclude clarification, for guidance is in the hands of Allah. A Muslim must convey what they have been commanded to convey. Therefore, you must clarify the ruling on a woman traveling without a mahram (definition), while exercising wisdom, as the Sunnah (definition) has warned against it. If a woman travels with a mahram and reaches a safe place, his continued presence with her is not required. However, the situation you described, involving the heedlessness of Muslims and the prevalence of innovations in a non-Muslim country, makes it inconceivable that the place would be safe.