In your case has two parts
- Marriage with a Muslim women
- Flat and future living with that girl
Marriage :
A marriage is a legally recognized union of a man and a woman, which thereto raises rights and obligations between them. Under the Special Marriage Act, you and your spouse could have married without changing your religion. Registration of marriage is a mode of proof of marriage. On adducing the registration certificate of marriage, it may be presumed by the law that marriage is deemed to have taken place.
If your girlfriend has been detained by her parents, you can apply to the High Court for her release from the shackles. When a person is of legal age of majority, it is against the law to hold against their will. They can also be prosecuted for illegally restraining her. So file a Writ of Habeas Corpus .
A police complaint can be filed against her parents for illegal detention against her wishes. The fact of love and willingness to marry each other should be emphasised and the apprehensions of danger to her life to be insisted on in the complaint. In the event that the local police are not cooperative, the higher official may intervene and direct the local police to retrieve the girl from illegal custody.
Flat :
Registering the flat already in your name will help you in the future.
Please understand that marriage is a sacred union of two individuals. Conversion of religion of a non-Muslim without any real change of belief in Islam and only for the purpose of marriage is void.