There must be an indemnity clause in your sale deed to indemnify you in case any claims are made by third party 

2) in case any legal proceedings are taken and sale deed is set aside you can sue seller to recover your money with interest 

See firstly the suit/complaint is time barred though in case the title of the property is disputed and A is not clear legal heir a adverse order can be passed. 

It is very difficult to be sure of how many legal heirs to ancestral property. 

One option is to give publication in newspaper regarding your intention to buy the property with a request to lodge claim by any person having any lawful interest in the property.

Collect encumbrance certificates.

Get the history of past transactions and legal heirs to the property embodied in the sale deed as a disclosure by the seller.

Take indemnity bond from seller for any title disputes.

Last of all you should know that if the seller having no clear title to the property knowingly sell it to other party having no knowledge of it, such sell shall be a fraudulent sell under criminal law and you can take criminal action against the seller besides civil remidies.

Yes it can be a problem if he is not a legal heir and if the same is challenged

Now you need to get mutation papers of the property from 1950 onwards till  date. And you have to check family history of the owner and who was real owner of the property.

Whomsoever is speaking false you will come to know. 

Better u seek the detailed consultation by disclosing complete details in this case, so that exactly can be suggested to do. 

Yes if A is not legal heir in that property then court can order to set aside the sale of property and give the property back to legal heirs of the property owner. 

You should have checked the records before purchasing the land from B. Now the whole case is dependent on the defence of A 

The complaint before the Tahsildar is not maintainable especially if you have purchased the property by a registered sale/title deed.

Let the adangal or any other revenue record remain on anyone's name, they are not title documents hence you need not be worried about it in that context.

Let them approach court of law with a suit seeking the retrieval of their property based on the documentary evidences in their support which can be challenged by you based on the documentary evidences in your possession and support.

If summoned by Tahsildar in this regard, you may call on his office, give a statement that you have purchased this property by a registered sale deed, hence if at all they have any grievance they may have to approach the court of law only and not the revenue department or the Tahsildar and request the Tahsildar to advise/direct  the complainant to approach  court for relief and remedy.

Dear Sir/Madam

The accused party has to file a case for declaration seeking cancellation of sale deed and tahsildar cannot pass any orders in respect of lands purchased by you under registered sale deeds.

To protect your position you may file a simple suit called suit for permanent injunction restraining the objectors from interfering in your lawful and peaceful possession.

IF A is not legal heirs, your property is at risk. It is general law that before purchase buyer must to title verification.