Is the husband responsible for his wife (and thus sinful for her transgressions) during the contract period and before cohabitation, especially when there are actions that contradict his religious opinion (such as not wearing the niqab and traveling without a mahram) due to study circumstances?

Once a man contracts a marriage with a woman, she becomes his wife. However, her obedience is to her father as long as she remains in his house, he has not consummated the marriage with her, and the dowry has not been handed over to her. If you have contracted marriage with this girl but have not consummated it, then you do not bear the sin for her transgression of the Shari'ah, but you must enjoin good upon her and forbid evil.

It is better for you to expedite the consummation of the marriage so that her obedience becomes due to you and her family no longer has authority over her. Otherwise, hasten the marriage contract along with continuously advising her on the aforementioned matters and clarifying the truth therein.