As you have legal title as a bonafide purchaser of the said property, you can file a criminal complaint against them at your earliest as delay is dangerous in criminal matter
and you can also file a civil suit against the builder ( proposed defendant no 1) and the person to whom the builder sold your flat after you (proposed dedendant no 2) for the cancellation of their sale deed.
as soon as you file a civil suit, lis pendens will be filed at sub registar where you registred your sale deed.
you can also be known of title and other detials of the said property by having search report of the same from sub registrar office.
you can inform the same to the financial insitutiton.