1) If you have extra money left over after one year or on the day your Khums date passes you have to pay Khums on it. If it’s apart of the expenses then you spend it on the expenses. Khums is just on the surplus, that includes student financing money. My advice is that if it is surplus return it to the institution that gave you the money.
2) If you start working yes you pay for all that is extra
3) If you are in debt you can’t go to hajj according to Sayed Sistani