1. You are already in right track.
2. Did your father file any application seeking grant of probate of your grandfather's will? Without grant of probate, will has no value.
3. You have already filed the case and passed so many years and the case is heading final stage now.
4. It will be illogical to succumb to your uncles illogical demand now.
5. He has filed the partition suit which you shall have to contest fittingly submitting evidence in support of your claim that the said property was purchased by your grand father from his earnings and not from the money given by his father. even if your uncle can prove that your great grandfather had provided some money to your father (which will be impossible for him to prove), your argument should be that it was gifted to your grandfather by his father.
6. You should now pursue your case vigorously without succumbing to the pressure of your Uncle.