1. Since Rupa has relinquished her rights in the property by a registered deed, she cannot claim her share in the property now without cancelling the registered release deed hence her suit for partition is not maintainable.
Whether you wait for three years or more than that since she has already filed a partition suit, this limitation will not be counted, hence you can file a written statement mentioning that she has no rights in the property since she has already executed a registered release dd relinquishing her rights and can produce the photo copy of the deed before court along with your written statement without waiting anymore time to file the same.
You have to defend your interests on the basis of the documentary evidences to support your pleadings.
2. The limitation will start from the date of its execution by the releasor, her sons do not have any role to play in this limitation part of the registered release deed because they do not have any rights in the property during the lifetime of their mother.
3. Rupa's children cannot make any statement before court in that manner because they do not have any rights in the property at least not during the lifetime of their mother.
Therefore whether they are in the knowledge or not about the release deed is not relevant to the suit filed by their mother.
They are not even party to the suit hence their application will not be entertained by court for any reason.
If Rupa believes that the release deed was obtained by A and B by fraudulent means then it is she who has to prove the same before court while she makes an additional prayer for cancellation of the release deed, hence without seeking cancellation of the deed, she cannot bring this subject to the main suit for partition, because she cannot deny the execution of the release deed.
4. The position of law is very clear and it has no room for your imaginary questions hence there is no question of worst scenario.