1. In the given burden of workload, the sub registrar has no time to go very deep into the title confirmation of the seller, especially if this is an inherited property. Besides, such works are carried out by the crooked brokers corrupting the sub registrar, hence such grave mistakes may tend to happen. The solution for the aggrieved is that he can submit an application to the District registrar venting out his grievances with the support of documentary evidences and request for cancellation of the registration or he can file a suit before civil court for remedy.
2. The remedy for this has been explained in the above answer. The so called void transaction cannot be cancelled by the sub registrar.
3. Since the property devolves on all the legal heirs/successors in interest, the sale of entire property by only one of the legal heir or few legal haiers without taking consent from other legal heirs is not valid in law, the aggrieved shareholders can file a suit for partition and also seek for declaration of the sale as null and void in the same suit.
4. There are plenty of case laws in this regard.
5. If you file a suit for partition you can seek the reliefs of declaration of the sale deed as null and void as well as an order of injunction against the purchasers as additional reliefs.