1. Did you make a list of gifts which were exchanged? Did the girl's family give you cash? You are liable to return all the gifts which were given to you at the time of engagement. It is your own case that apart from gifts, money also exchanged hands. If this be so, the girl may file a dowry case against you which may unnecessarily enmesh you in the rigours of law. You may be able to rebut that gifts exchanged were not dowry, but it will be difficult to prove that money was not taken as part of dowry.
2. Flight expenses were spent by them out of their own free volition, so you are not obligated to reimburse them. However, this does not prevent you from returning them on your own accord.
3. You are not obligated to refund the expenses incurred on marriage and catering as these were spent by them on their own accord.
4. Girl's family is bound to return the gifts given to them. In the event that either party refuses to return the gifts to the other, the latter can file a case for recovery in the court.
5. The return should be documented to prevent legal hassles. Consult a local lawyer immediately.