no,viren is not the stalker.viren is doing wierd things which r shown to us in the serial.but viren's path is different and the stalker's too.the stalker's aim is to throw nivedita out of the house but not to kill her.
i seriously have a doubt that it is jawahar becoz when chanchal told that viren is ready to give his life for nivi, he might have thought of other ways of throwing her out by proclaiming her insane.and for all this he took chanchal's help.viren's aim is nivedita and her love.moreover why should he harm her!now dadaji is perfectly better than before and everything is fine about him.viren knows nivi better than herself and that she will not do any harm to anyone and why should a person take revenge on such a scary girl!!!!!!