Is it permissible to delay pregnancy using birth control pills in order to pay off a study loan, and what is the ruling on working in a mixed-gender environment while dressed modestly but without an abaya or niqab?

First: Usurious loans are forbidden, but you bear no sin if you were unaware of their prohibition. It is recommended to repay the loan early to rid yourself of usury.

Second: It is permissible to postpone pregnancy with the mutual agreement of the spouses for a valid reason, among which is working to pay off debts.

Third: The شرعي hijab (definition) has specifications that must be adhered to, and the niqab is obligatory.

Fourth: Unlawful mixing (اختلاط) refers to the gathering of men and women without شرعي controls. If you are compelled to work in a mixed environment, adhere to the شرعي controls such as hijab and lowering the gaze, and search for work free from mixing.