At this point, as per me, chances of who the stalker can be:
1. Sid: 10%, logic is his character is not that bold to stalk anyone.
2. Viren: 50/50, because till this point, he has done everything good for Nivi, but we are yet to know his past and whether he has any other motif. Thus 50-50.
3. YD: 0%, as she is a loud character, with no ability to do anything other than shouting and blaming.
4. NT dadi : 0%, as its like suspecting God, as NT dadi is mother cum God to Nivi.
5. Vandana: 10%, as no sound mtif is there.
6. Keshav: 0%, he is an angry father. From the previous generation also, I have seen fathers, who did not see their daughter's face, if they went and married against the family. Its very common, but that does not mean he will stalk Nivi. Also, I feel he is a weak character and unless backed by someone, he is not possible.
7. Yash: 0% (dont think any logic is required).
8. Viren and Vandana: 10%, and in this case,I'ld say they have positive motifs, to make Nivi strong, as after every stalking Vandana comes to console Nivi. Also, Viren, wants to teach his own parents a lesson by exposing their involvement in Bhuban murder and Nirmala's suicide.
9. Sid and Vandana: 10%, they want to seek revenge against Nivi. But again both are too weak as to harm anyone.
10. Nandu and Lalji:10%, being accomplices.
11. Divya, HL, CC and JW: 90%: this combination holds the most logic. Divya and HL are greedy and selfish, while CC and JW are criminal minds. CC, JW are the schemers and Divya, HL are made the instruments by JW and CC for their own benefits. Divya, HL want to get rid of Nivi from Sood house, CC, JW wants to get Nivi out of Viren's life and also get a proper hold of the property through Viren by framing Sid as the stalker eventually (Viren being oblivious of his parents wrong doings). Even HL and Divya might not be aware of this second plan of CC and JW about framing Sid. HL and Divya just wants to get Nivi out of the Sood house.
If you want, you can analyse accordingly.