Is using my father's credit balance at a pharmacy, which resulted from insurance-covered treatment invoices, to obtain medications and medical supplies not mentioned in those invoices considered a violation of religious law, especially since I withdraw my own medications from this account with my father's knowledge?

It is not permissible for a Muslim to resort to trickery and deception to unlawfully consume people's wealth. Therefore, what your father takes in medications, in violation of the terms between him and the insurance company, thereby obtaining what he is not entitled to, is not permissible for you to take. You must advise him to desist from such actions.