Can signed letter from in laws help if in future wife file false case?

As we can see, in our opinion you should firstly safeguard yourself, your parents and your relatives and collect evidence. This basically involves recording your conversations with your wife and her parents. Just be polite and record all your conversations. This is essential as these recordings will help you prove the complaints she files against you as false and baseless.

Try and record the threatening conversations wherein she is threatening you to file false complaints etc. 

Apart from this, intimate the Police about the facts of your case. You should intimate the nearest Police Station about the facts of your case in writing stating that your wife and her parents may file such false case against you and your family. And if she is threatening you with false complaints then you can even write a complaint to your nearest police station, detailing about blackmailing, her false allegations and her unscrupulous behavior. And request in your complaint to make her stop the threats and abuses immediately. 

Yes, you can get the statement signed but do get it signed in the presence of 2 neutral witnesses or else the same can be said to be manipulated and can be contended in the Court of law to be signed under coercion.

And with respect to divorce, the petition for the same cannot be filed before the end of 1 year of marriage.