There is no need of witness during Nikah according to Shia Jurisprudence but yes at the time of Divorce there must be two just person.
There are two different conditions
Issue 2382: If Nikah of a woman is pronounced to a man without her consent, but later both man and woman endorse the Nikah, the marriage is in order.
In this situation it’s better to repeat the Nikah.
Issue 2383: If the woman and the man, or any one of them, is coerced into matrimony, and they give consent after the Nikah has been pronounced, the marriage is in order, although it is better that the Nikah be repeated.
Verdict of Ayatullah Sistani clearly says that if any of man or woman was not forced but one of them were not agree by heart and if later if they agreed their Nikah is valid because as soon as you started living with your husband that is your agreement.
None of the Islamic Shariya deals with what a person has intended deep inside his/her bottom of heart. All the Nikah is based on the consent of a woman when her Wakeel asks her about her marriage with such Mahr and person with whom she is going to be marry then her Nikah become valid.