1. They may be ok to sign any document though.Would that help in any way ?
This would not help to solve the issue in any manner
2. I had word earlier with her parents(in 2015) and at that time they said they accept their fault and they ok if i moves ahead in my life and get remarried.but when i said lets close it gracefully for that they said dont pull us to court and all now .They said i am free from their side however I am afraid what if they change their stand later inspite other family members knowing the situation ?
If she is not willing to agree for mutual consent divorce, then you can file a divorce petition on the grounds cruelty and it can be contested divorce, if she accepts the summons and refrains from appearing before court, then she will be set exparte and exparte divorce can be granted which is also without shedding any blood in the legal war.
3. What if they don't response anything at all to either mutual consent divorce or contested divorce.
You can proceed as suggested above.
4. Secondly what could be legal implication if i didn't close it properly through court,she moves ahead in her life and i moves on in mine considering all senior common family members know about this situation and cause.
Both will be doing blunder mistake because s per law there has to be legal divorce without which there can be no second marriage too.