Mr Ranjit, It okay your father has sent a letter through registered post. If you are that your wife is alive, and chats with your friends or exchange messages, then try to keep her chat record, save the chats. That will be a proof with date & time.
For filing RCR you need to file a petition , But for filing RCR you need the proofs that you are married couple, ie you need photographs or marriage certificate etc. You can contact a good & affordable lawyer . You can discuss with your lawyer and you can file your case under appropriate section.
Well don't call your inlaws, try to send them letters through registry post, write normal letters , no threatning, nothing abusive, just a normal letter. Yes you can also take help from a good & trustworthy mediator. You can put your point that, if their daughter is not alive then please give the death certificate, and if not then either think & give chance to your marriage without breaking it bitterly, or if they don't want to continue then mutual divorce is best. But things should be done in a proper & genuine way respecting law.
My email id is shreyad71@gmail.com
Thankyou , all the best