Hangmy:
I read the novel but I must have missed the part about TY convincing YH cultivating demon path because he has a motive of getting back the HPS and Neidan. Is that really the case? I only remember she did it because she was tortured, lost her Taoist cultivation, her mind’s eye was destroyed, no?!
You're right, cultivating the demon pathway was solely YH's decision; her xian cultivation was gone and she wanted to avenge Zi Chen's death. (The mind's eye element, however, is from the drama, not the book). TY never intended to take back the HPS or niedan from her (he even tried to keep her from knowing she had his niedan).
An excerpt from after she's rescued from the Chen Xing prison:
"Yan Hui fixed her eyes on [Tian Yao]. The gleam in her eyes were like polished silver, flickering with cold light: "Once I've healed, I want to cultivate the demon pathway."
[redacted]
Tian Yao looked at Yan Hui for a long time. Then, he nodded: "Okay."
There was no questioning nor evasion.
She wanted to heal her body; he would help. She wanted to cultivate the demon way; he would teach."