Legal heir certificate

( 1 ) Yes, after demise of A, his property rights devolve to D1 and D2 and being legal heirs of A, D1 & D2 have  right to execute sale deed.

( 2 ) Heirship certificate obtained by competent authority is sufficient for the purpose of sale transaction.

( 3 ) Grand children of D1 and D2 have no right to challenge thesale transaction. 

If the property transferred to A by a Will was acted upon by A, i.e., by transferring the revenue records to his name by enforcing the Will, then he becomes the absolute owner of the property he acquired through the Will.

In the event of his intestate death, the property lying on his name shall devolve on his legal heirs.

They have to get the property transferred to their names in the revenue records after which they can execute the registered sale deed in favor of the prospective purchaser jointly. 

The legal heirship certificate can be obtained through the renvenue depatment (tahsildar).

As the property devolved on D1 AND D2, they will be considered as absolute owners of the property hence any claim made by their next generation children will not be legally maintainable.