Is zakat obligatory on a sum of money my father deposited into my account for my studies in Germany, given that a large portion of it is a debt owed by my father and a hawl (year) has passed on it? And if zakat is obligatory, then upon whom is it obligatory: me or my father? And is it sufficient if one of us pays it on behalf of the other?

You must pay zakat on the money if it reaches the nisab and a full lunar year has passed since it came into your possession as a gift. The zakat is obligatory upon you, not your father. It is permissible for you to authorize your father to disburse it from his own money. However, if he disburses it without your knowledge or authorization, it will not be sufficient for you due to the absence of your intention.