Actually they both are important to Payal's recovery. Durga has her identity to protect. She can give Payal medicines, sister like care, can take her outside everything , but she cannot give Payal motivation. A need to get herself cured. I mean Payal identifies Durga with Nithya but Durga denies she is Nithya. So the girl does not have anyone of her own. Also Durga cannot bring back the beautiful memories for Payal. The memories Payal has about her parents, her sister and her protected life before the rape. Reason she has to protect identity.
Now along with medicine and care if a motivation and beautiful memories are given recovery will be easier. Dev is Payal's connect with the past. The only person she knew and trusted in the past. That's why she is responding to him. Dev can take the old photos, videos, CDs memories everything to Payal. And he did give her the biggest motivation to get cured. He will bring her Nithya di in front of her on Nithya's b'day.
You cannot discount that motivation. The need for Payal to look healthy and happy in front of her sister. She will want to do that. Because now she believes her sister is out there. Her sister will come. Someone close to her is alive not dead. Such a force can do miracles for the girl.
Yes Dev's motive might be to reach Nithya but it is not by destroying Payal he is trying to reach her, it is by trying to cure her only right. Yaa Sakshi reached Payal through Dev. But did he do that intentionally. How easily he guessed just from Durga's eyes the attack was on Payal and he was the only one who did came to hospital post that incident.
Sakshi had shut up Payal, she tormented Payal to reach Nithya. Dev is using the reverse way to make Payal speak up, make her recover what if the motivation is Nithya.