Since this is the first lakorn starring Pinky that I am obsessed with, I feel I need to put in my two cents worth. I've read some of your comments and to a certain extent I do agree with them, but as I keep thinking about it the writers probably had a good reason why they're having Kwan come in during the last parts.
Maybe I'm just thinking too much into it, maybe they aren't even going towards that route. Weir may still love Kae Kai because he remembers his past life, but it's not the same. He's technically a different person living a new life - it just so happens that he remembers his past. It's hard to explain, but I'm treating it as two different things. Root was Root and Tuamon is Tuamon - who was reborn to live out his life with new experiences and a new love - who happens to be Tua Lek. A phrase they kept repeating was that what happened in the past was in the past. You can't change it, but you must move forward as best you can.
He may not know it yet, but he has fallen deeply in love with her to the point where he can't live without her - example was in episode 4 where he jumped into the river because he thought Sang captured Tua Lek.
Many have guessed that Kwan is the old man's sick daughter. This would make sense to me if this were going to end how I think it may end. Since it was Kae Kai's energy that brought On-In to life, once they solve the mystery of who really killed Root and Kae Kai wakes up - She'll have to take back the energy she passed to Tua Lek. Without the human energy, Tua Lek will have to go back to her original form.
However, she's fallen in love with Tuamon and loves him too much to leave him. At the same time, Kwan is dying from her illness and the only way for On-In to live in the human world to be with Tuamon is to possess Kwan's body.
Kwan taking over for Pinky - kind of like saying that Tuamon didn't love Tua Lek because of her looks, but because of what's inside - her naivety, her innocence and honesty was what he loved about her. Therefore, it shouldn't matter if Tua Lek looks like Pinky or if Tua Lek looks like Kwan, she's still the same person on the inside. I know it's too cliche, but I find myself being able to mentally handle the lakorn better if I justify the switching of Nang 'eks in that way.