Regarding adivasi land purchase

The threats of the buyer may not work against you, as he himself will be landing in trouble for a benami transaction.

Knowing that it is Adivasi land and sale is barred you have purchased the same.  Your applying for permission to collector is itself a proof that you are aware that land transfer is subject to certain approvals.

Better contact local advocate and take his guidance as you have on your own invited and landed to unnecessary controversies and it is not possible for any expert to guide you without knowing complete facts with such documents.

Please check whether the land allotted by State/ Local government to an adivasi can be sold/purchased by another adivasi ? I think it can neither be sold nor purchased but for exclusive use of allottee ?

However, if some one else claims to have purchased the land you can file either suit for specific performance (if you are ready and willing to pay balance of consideration) or suit for recovery of earnest money paid to the vendor (seller).

Consult and engage a local prudent lawyer to appreciate facts/ documents, guide and proceed.

How can 1 check whether the adivasi land can be sold or purchased to any other adivasi?

You can entrust this to an advocate and make a search of such sale transactions in the office of Sub-registrar over a period you wish.