I think its Nakul and not Bhalla. In the 27th September story, Rajjo tells Bui that Bhalla came out of nowhere and landed in front of her. it means this is Bhalla & Rajjo first time encounter with each other, so how they both planned in advanced to separate Simbhi.
In my opinion, Nakul is the master mind because Bui and Rajjo are his relatives and can be easily duped into his plan of revenge towards Simbhi or Sethi family as a whole as they are naive due to village background.
Nakul wants to seek revenge because :
- Simi humiliates him in front of everybody during their supposed wedding by declaring Abhi is her husband.
- Abhi wins Simi's love and Abhi does not apologize (not sure what's the reason why Abhi must apologized) during the trial at court
- Abhi who is much younger than him out do him and this shaken his self esteem
- Simbhi alliance
- Nakul was humiliated more than once by the Sethi's; slaps from Mili and Simi, cheated by the Sethi's to accept Neetu's who is pregnant by his driver as his sister-in-law, Neetu's questioning him about Omi's medicine prescriptions, Rajinder pickering on hos shirt and pushing off his present when Mili withdraw FIR, Mili winning the FIR case, and as a result he was brought to jail and downgrade his reputation as a rising politician, publicized and photographed in public etc
Just my opinion.