IT is mandatory to probate the WILL in the areas of Bombay, Calcutta, and Madras.
It is not mandatory to probate the WILL in other areas.
It is not mandatory to register the WILL. However WILL may be preferably registered. The registered WILL is not easily set aside atleast on counts of authenticity. And in case witnesses tend to be hostile or also deceases the interest of beneficiary (ies) remain protected.
There are many threads on similar query that you can SEARCH in SEARCH option in threads, Articles, Files etc etc at LCI also e.g;
GO thru;
https://www.lawyersclubindia.com/forum/Digital-will-vs-registered-will-178876.asp
https://www.lawyersclubindia.com/experts/Regarding-will-459536.asp
https://www.lawyersclubindia.com/experts/what-happen-if-beneficiaries-of-will-predeceased--663796.asp
The WILL should just be valid. Unregistered WILL can be acted upon.
The last valid WILL prevails.
If a qualified doctor say; Psychiatrist, certifies that testator is of sound mental health IT is better.
If clear descripttion of boundaries is given in WILL to dispose immovable property IT is better.