The facts mentioned in your post is quite impressive that is shown chronologically has impressed us to reply you lawfully as per precedence and provisions of law.
We herein reply your lawful questions as under.:-
1- it's nice you have GPA of all 15 person but trouble is that it is not registered with the competent authorities sub-registrar of property at registration office. GPA is not enforceable as per Indian Registration Act. Hence in case of suit against your opponents you may loose the case on the issue validity of legal document befor the Civil Court.
2- It is nice ground for you to obtain injunction and declaration of title against your opponents based on payment receipts evidently accepted proposals for sale and transfer of land as per Indian Contract Act and Transfer of Property Act. Indeed it favours to you for Injunction and suit for declaration of title ,interest, occupation and possession of the same under Specific Relief Act.
3- Marking of documents before the Court does not become admissible evidence as per India Evidence Act 1872. You will gave to prove it however you would get set back at the trial before Civil Court because of non- registration of GPA .
a) Good for you 3 died.
b) Problem for you because of non- registration of GPA which is compulsorily be registered under Indian Registration Act.
c) Positive sign for you that 7 people are loyal to you .
Option
Please approach good Advocate ask him to file suit for the injunction and declaration of title under section 34 and 36 of Specific Relief Act and order XXXIX Rule 1,2 and 3 of C.P.C. 1908.
They may get the suit dismissed on the ground of forged documents and false suit .