My wife's sister once mentioned that she knows good lawyers in India and she asked my wife to drag me to India so they can take care of me legally in India. If we are granted a No-Fault divorce, can my wife's mother or sisters contest a No-Fault divorce granted by US court assuming both husband and wife were present during the judgment just to harm me and my parents/sisters or to extract some more money?
You should understand one thing that the marriage was held between yo and your wife similarly the dissolution of your marriage by a decree of divorce also happened between you both in both your presence.Therefore there is no room or chance for an outsider to contest or challenge the 'no fault divorce' granted in US and as you said that it is valid in India, it cannot be contested any case filed in this regard by a third person shall not be maintainable, so dont give in to the pressures of her sisters or any other irrelevant third person's grumblings.
If I can prove that she has been committing mental cruelty by threatening to commit suicide and quote me as the reason or asked me to take poison and end my life and even hitting me at times, then the court may grant fault divorce. Since she will be at fault, it will not be a MCD but she will be present during the court proceedings. In this case, will the divorce be valid in India?
Yes , this can be treated as contested divorce, which is very much valid in India just like a recognition to MCD.
3) Considering the above case as true, can my wife's mother or sisters contest a Fault divorce granted by US court assuming both husband and wife were present during the judgment just to harm me and my parents/sisters or to extract some more money?
They are all third persons or outsiders to your marriage so they do not have any right to interfere in your personal affairs. The divorce is your personal affair between you and your wife, interference by a third person is not permitted, so they cannot challenge the divorce case for any reason until and unless your wife is insane.
They cannot do anything in this by themselves, if they indulge in any such activity they can be booked for illegal acts and extortion offences.
If she can claim/prove that I have been at fault of some sort, then the court will grant fault divorce. Since I will be at fault, it will not be a MCD but she will be present during the court proceedings. In this case, will the divorce be valid in India?
The answer to the question two shall be applicable to this too.
5) Considering the above case as true, can my wife's mother or sisters contest a Fault divorce granted by US court assuming both husband and wife were present during the judgment just to harm me and my parents/sisters or to extract some more money?
The question repeated, see the above answers.
6) While the divorce process is initiated in US, what are the ways my wife's mother or sisters can try to harm me or my parents or my married sisters in India?
They cannot harm or try to harm your relatives by any means except indulging themselves in physical violence against them.