The proeprty is on joint names and also she is one of the applicants for the home loan.
Therefore even if you have discharged the home loan in full, as far as the bank is concerned t you both are the title holders hence the documents pertaining to the proeprty can be handed over to both of you after you both sign the papers held by the bank to acknowledge the return of title documents.
You will not get any relief from supreme court or RBI ombudsman in this regard because what you are insisting to the bank is an unlawful act and you cannot claim it as a right in this illegal act.
You have reconcile with your ex-spouse and go forward to retrieve the property documents, even then since the property is on the joint names, you may not be able to have an absolute and clear marketable title to the proeprty until she relinquishes her rights in the property by either selling her share in your favor or executes a registered gift deed in your favor.
The proposed declaration suit, if disposed after very long legal battle, would only declare your title based on the convincing document you may produce before the court, however that will not be a binding factor to the lender bank to return the propeerty related documents to you alone without her signature.
You may consult an advocate in the local and try to sort out the issues with her and arrive at an amicable solution instead of stretching it into a very long drawn legal battle.