1) can these people misutilise these documents to sell this plot which is in my dad's name?
If the so called prospective buyers are not genuine buyers and their intention is to defraud you or do some illegal activities against you for wrongful gains, you cannot come to know about their such evil deeds until the act is done, hence as a safety measure you may give a newspaper publication stating that the so and so buyers who pretended to buy the properties in the schedule given therein are suspected to do some mischievous activity in this regard or involving in a scandalous act to defraud the the prospective victims hence you being the original owner shall not be held responsible for any such claim for the acts done through the said fraudsters.
This step will protect your interest in the property even if they try to misuse the copies of the property document held by them.
2)What course of action to take in order to safeguard/protect the plot and to make sure that this party cannot cheat us.
The suggested measure in the above paragraph may be followed besides your own ideas in this regard.
3) Will approaching the society and giving them a letter in writing by the owner (my dad) be useful?
Such additional measures also will be appreciated which are in your own interest.