NRI divorce

In strict Legal terminology, annulment refers only to making a voidable marriage null; if the marriage is void ab initio, then it is automatically null, although a legal declaration of nullity is required to establish this.

Grounds For Annulment
The grounds for a marriage annulment may vary according to the different legal jurisdictions, but are generally limited to fraud, bigamy, blood relationship and mental incompetence including the following:
1) Either spouse was already married to someone else at the time of the marriage in question;
2) Either spouse was too young to be married, or too young without required court or parental consent. (In some cases, such a marriage is still valid if it continues well beyond the younger spouse's reaching marriageable age);
3) Either spouse was under the influence of drugs or alcohol at the time of the marriage;
4) Either spouse was mentally incompetent at the time of the marriage;
5) If the consent to the marriage was based on fraud or force;
6) Either spouse was physically incapable to be married (typically, chronically unable to have s*xual intercourse) at the time of the marriage;
7) The marriage is prohibited by law due to the relationship between the parties. This is the "prohibited degree of consanguinity", or blood relationship between the parties. The most common legal relationship is 2nd cousins; the legality of such relationship between 1st cousins varies around the world.
8) Prisoners sentenced to a term of life imprisonment may not marry.
9) Concealment (e.g. one of the parties concealed a drug addiction, prior criminal record or having a s*xually transmitted disease)

I do not think the marriage cannot be annulled after the witnesses have signed, if the above conditions are met with. Signature of witness has nothing to do with annullment of marriage.