Think of the repurcussions of taking the legal route. IMHO, do not seek a "very passionate,honest,selfless,brilliant and experieneced lawyer", instead seek a family counsellor and or Clinical Psychologist first.
There may be medical issues pre-disposed, ie the wife/mother of 3 year old may be pre-disposed with mental issues prior to marriage or genetically. It is possible that the joint/extended family did play a role in triggering or aggravating a pre-exisiting mental condition. A clinial psychologist can sit with both parents and try to educate them on the pros an cons of living a family life versus living separate and in the worst case making the decision to dissolve the family.
If counselling/reasoning/education does not work, then both parents can go for a mutual separation and continue to co-exist in separate homes but yet continue to love and care for the child. May be the family ( father - mother - child ) living away from the inlaws may solve the issue or reduce the stress. Have you looked into these internal issues before considering DV.
Imagine if learned Ambika and Roshini B's suggestion leads the mother to get involved in litigation and the whole ordeal of going to courts, appearing in front of judges triggers more severe mental issues and the mother commits sucide or harms the child or becomes a total mental wreak.
Would Meera Neeraj/Ambika/Roshini adopt that 3 year old and give the child love and care ???
Lawyers and courts cannot bring peace to the family or love a child - only loving and co-operative parents can.
A smart lawyer, IMO is one who can settle the matter out of court. Once again, think of the repurcussions - DV may create unknown / unseen enemies for life. Do you want to bear that cross for the rest of your life for filing DV. What if filing the DV turns the innocent husband into a psycho - the psycho finds who sowed ill thoughts into his wifes head and then the psycho comes and gets you ??
Now, lets look at it the other way. The husband is truly the dominant aggresor ( it takes two to tango ). Over the period of 8 years, there would be several witnesses, evidence of abuse etc. The question arisies, has there been any escalation of the issue to family, friends, neighbors, counsellors or this DV thing has come up one fine morning.
In your post, you have mentioned "defamation suit". What has the wife/women said or done that is making the husbands parents defame her ? Did she sleep with the neighbor or is indulging in adultry ?
Behanji, zara kahani khol ke batao na. DV me kohi jita nahi , koi haraa nahi - kaun haara - woh teen saal ka batchaa. Kyon paap jhelna chahate ho behan. Aapko zindagi se pyar nahi hai kya ? Kabhi kissise mohabbat kar ke to dekho na ? Apne ados pados ke batchon ko dekho - kitna pyar se khelte hain - Jis batche ko baap nahi, woh badaa hokar chor/daku bankar kahin aapka nuksaan na pahunchva de.