Will deed validity

Your grandfather X died in 2019, so B, C, and D have no right to sell the property before X's death, making the sale invalid.  And you have not mentioned who are the beneficiaries as per the Will of your grandfather.  What are the contents of the Will?  Consult a local lawyer with a copy of the Will for further guidance.

The legal heirs of your deceased aunt and your paternal uncle are reported to have transferred their share in the property as per the Will but it is actually invalid because the Will did not come into force as on the date of transfer of their share.

However as your father, who is one of the beneficiaries of the Will has been alllotted with a share in the property died even before the Will came into force, his rights over his share will automatically devolve on his own legal heirs, therefore the refusal of the Village officer to transfer your father's share to his legal heirs by mutation is illegal and against the law of inheritance.

You can give an applicaiton to this effect and ask him to refuse to do so and give the reasons for not doing it in writing after which you can prefer an appeal before the Tehsildar, District collector against thsi order of the village officer.

Village officer is not knowing the law hence he should be taught law by the officer above him

They had transferred the shares (area 2/3) mentioned in the will to there name believed to be as gift deed without cancelling the will deed.

The Will cannot be cancelled if one of the beneficiares dies before the testator.

The beneficiary's inheritance will be kept within the deceased's estate and distributed to the surviving beneficiaries.  

A General Power of Attorney can be prepared authorizing the son to handle and manage all properties pertaining to the individual, though the details in regard to some of them are unknown. However, for the known properties, such as the fully owned flat and the one where he is a legal heir, more details should be included.

The GPA can be generalized to apply over any property, whether wholly or partly owned by the individual. A generalized GPA is not per se invalid for Indian law allows a GPA to encompass general powers to all present and future properties owned by the grantor. However, upon registry, the sub-registrar may insist on some sort of identification of properties for clarity. If the particulars are not known, then a general clause will suffice. The GPA would then state that the GPA covers all the present and future properties over which the person has rights or claims.

Basis of a GPA:

Grantor's and attorney's particulars.
The nature of powers being granted.
All known properties are a general clause with respect to unknown ones.
Signed and attested; and, if the individual is abroad, then attestation by the Indian Embassy.
Thus, it's not necessary that all the information of every property should be incorporated for the GPA valid to be signed; however, adding a general clause is preferable to have wider coverage.