(a) That's right. This contract could definitely be set aside on grounds of duress. The law will not tolerate threats of physical violence (or the actual infliction of physical harm) as a means of getting another's consent to enter a transaction.
It does not matter that, as in the present case, the threats are made against B's wife rather than B himself. It is well established that a contract can be set aside on grounds of duress if the threats of violence are made either directly against the contracting party, or against a person who is related or close to them.
It also does not matter whether the threats were the only reason for agreeing to the contract - it is enough for the plaintiff to prove that a threat of harm was one of the reasons for which they entered into the transaction.