The builder is stated to have forged PoA and sold 35 units on behalf of benami (unknown owners) which you (buyers) do not want to challange and cancel, is it so ?.
Further you have stated that the builder has used the fake PoA to form an association or Society and took over as elected Chairman. You want to challange the election/ selection of the chairman without disturbing the sale transactions, isn't it ?
There are two contradictory statements in the facts posted, be clear.
It is better to contact and consult a local prudent lawyer for appreciation of facts / documents, forming proper opinion, professional advise and necessary proceeding. However, if you are located in Delhi / NCR and feel so, may contact me on appointment.