Hi
I) Your title, ownership and possession is perfect and you do not need to worry at all about the WILL getting probated and Mr. X's legal heir's claiming the property from you on the following grounds:-
a) You have purchased the property from the Mr. X. So Mr.X has executed the sale deed thereby conveying the property to you despite having knowledge of the WILL executed by his father.
b) Section 41 of Transfer of property applies to your case and concept of Ostensible Owner, Express/Implied consent by Mr.X for selling of the property to you claiming to have obtained the property from his father will apply and will be in your favour.
c) Leading case laws such as Mohamad Shakur v Shah Jehan, Satyanarayana Murthi vs. Pydayya are similar to your case and you can claim ownership, title, interest and possession based on above case laws.
2) Not only you have the sale deed, but also been in possession since 1986 and that too with the full knowledge of Mr.X and his family members.
3) So legally and technically, Neither Mr.X( even if he is alive) or his legal heirs cannot claim ownership and possession.
4) File a case of declaratory title in the court and establish your rights or alternatively contest the case in court if at all the legal heirs of X file a case against you. You will WIN your case (as per section 41 of transfer of property act)
Hope this information is useful