How to deal with probate case

My grandfather died in sept, 2009. He had two properties. He left behind his wife(my grandmother), 2 daughters and 1 son. The son is independent and my mother is a dependent daughter of my late grandfather. My grandmother mutated one of the property in 2014 in her name after both the daughters gave NOC, but the son didn't give. Now in 2017, the son has produced some kind of will of my late grandfather which is in his favour and has filled a probate case. Now my questions are: 1. We know the son might have forced our grandfather with that will as he has lied in the application he has given and there are mistakes in the will. How can we prove it? 2. What are the different ways to contest a will? 3. As my grandmother has already mutated one of the properties in her name, will the Will affect her mutation? Will she still be considered as an owner after probate? 3. How long it will take for the case to settle? 4. Will the probate case lead to further problems in our properties?