1. Legally, there is nothing that you parents can do, other than file a case of alleged cheating/fraud against you. The same will get quashed when the girl herself testifies that she has done it of her own free will.
2. Getting married is not necessary for any such safeguard.
3. Recommended yes, but it would probably make more sense to inform the parents itself that she has chosen to leave the house. It would save the parents from having to file a missing person report in a police station
Do keep in mind however, that the parents can legally disown her, thus alienating her from any of their assets/wealth