A new challenge

My father has two elder brothers and a sister who is deceased now.
My father purchased a plot of land in the suburbs of Mumbai using his money but his father's name in 1973.
He was short of Rs 10,000 which was paid by my paternal aunt Rs 5000 and Rs 5000 by her brother-in-law.
 
The plot was purchased with full cheque payment from my grandfather's account but with the cash deposits of my father.
My father later took a loan of Rs 1750000 from a co-operative bank for constructing a  building with commercial premises and mortgaged the commercial  premises for the same.

According to the mortgage deed the co-mortgagors(grandfather, aunt & her bro-in-law) have agreed to allot the ground & first floor to my father (the mortgagor) while the second and third floor's are not included in the mortgage.

The loan was duly paid by my father and all the prevailing taxes paid.The bank has misplaced our original property document.
There is no development agreement of any nature or building rights agreement.

I have the old & new property cards. The old cards bearing the names of my grandfather, aunt & her bro-in-law
while the new property card bears the names of my uncles,my father and the husband of my deceased aunt.
The property is divided into 8 parts.

Part 1- 6500 sq ft (commercial) in the name of my father.
Part 2-  871 sq ft  (residential) Aunt
Part 3- 1131 sq ft (residential) 1st Uncle
Part 4- 1166 sq ft (residential) Father
Part 5-  706 sq ft  (residential)  2nd Uncle
Part 6-  500 sq ft  (commercial) 2nd Uncle
Part 7-  200 sq ft  (shop) Aunt
Part 8-  200 sq ft  (shop) Aunt's bro-in-law

What I want to know is not how the other parts will be mine but on the basis of which document does the commercial area of Part 1 which is 6500 sq ft belong to my father? Since the Electricity Bills, Water bills are in the name of my deceased grandfather.
Since 15 years my uncles are taking Rs 8000 as 'consideration' amount from my father for the first & ground floor and want to put it in the legal papers; an affidavit which as per my knowledge holds no standing.
My father is paying 90% of the assessment and maintenance charges but in cash.

What are my chances in this situation?