Shanti...on the other hand...
Shanti, with the guidance of Lata, definitely had the opportunity, the motive, the necessary materials (the medicine bottle), and the fact they don't show up in scenes together anymore, meaning one of them could be off haunting up the house while the other is pretending as if all is well.
I'm sure you all know Lata's motives and have noticed her absence, so I won't go too into detail on that. As for Shanti, she has the motive - avenging her mother, and securing her brother and best friend's happiness. In addition, she's conveniently either not there in some of the scenes whenever the doll shows up, or in a position to easily not be noticed as the doll.
Today, for example! The doll shows up at the Burve mansion while Aniket gets hit over the head in front of the entire family. Even Lata is there. Survi is back in her own house with Karthik. And where's Shanti? Neither of those places. She had left Survi's place, and while there is little time gap, she's the only one from the main cast missing. Where else would she be? Chilling in the living room and having tea with Shrikant Kaka?
During the scene where Aniket goes missing, they show Shanti among the guests. But then, the shots focus exclusively on Karthik. There's enough time for her to go to 'kidnap' Aniket. And when the lights do come back on, she's right there, as if she never left. She could have planned it all out in advance. This is where my argument becomes weak, but if Lata is helping her, then this theory still prevails in the context of this scene.
Also, there was the dance scene. Shanti was the one reading the slips of paper. She was right next to the bowl. She had the perfect opportunity to slip in the note with Taayi's name. And not to mention, when she was reading off the slips of paper, the camera conveniently did not show Karthik's name on the paper when she read it. It showed everyone's names as she read it, but not Karthik's. What if the slip of paper didn't have his name? What if she made it up? And then the doll shows up while Karthik and Survi are dancing. It's perfect, isn't it? None of the attention is on her.
Even if Survi is the mastermind, I don't believe she was alone. She definitely had Shanti and Lata's support at the least.
Until I see solid proof, I won't believe Shanti isn't involved with the doll plot.
(Someone please give this girl more screen time because she's an awesome friend and sister and I need more of her.)