Dear Querist in relation to your question my advice to you is as follows:-
1. As per Hindu law, the property will be treated as self acquired property only. There are so many judgement of the Hon'ble Supreme Court and various high courts which have cleared on this concept which properties can be treated as self acquired and which not.
2. For being an ancestral property, the property should not be divided among brothers or sisters and it is passed upon descendents as one unit for three generations without any gap, only then a property can be treated as ancestral property.
3. As per your case, your property was divided by your grand grand parent and then it was inherited by your grandfather as being only son, so he will be considered as first generation and afterwards to your father, second generation but in the second generation it was inherited by your father solely, you are third generation but as it will be distributed among you as per your share, so it can't be considered as an ancestral property.
4. Your grandfather could legitimately done a will of this property as self acquired property, so it can be executed.
5. Yes, you can ask for partition of a suit property as per Hindu succession act.
6. Will can be a registered as well as non-registered document.
7. You don't have to execute the will, a will comes into existence as soon as the person executing it dies. So, the Will is in power and you just have to ask for your own share before a court in accordance with this Will through a partition suit. You can file a partition suit in this regard.
8. If this is a case, that you think the Will is forged or fabricated you can file a case against other persons for a declaratory suit in order to declare that Will null and void and pass on the entire shares in ur favour.
Lastly, my advice to you is you can claim your share.
Feel free to call.