I agree that Zefang was more pitiful than bad and his grandfather also wanted people to give him another chance if he returned to his body. In the end, Xie Cha gave up the body to Xie Zefang but he refused it. I get why it is controversial considering the age gap and relationship between both men but it seems Zefang himself had given up on life. You can try to look at it as someone choosing to donate their organs after their death and his grandfather was the best match. It may seem like a stretch but that is the situation presented here.
I'm not sure. Unless he isn't reborn but loses his memory instead and so they both go live in the immortal realm since flowers were blooming magically. 😋
But it's okay to post spoilers and theorise since you are using the spoiler. Yeah, I think the drama is getting interesting. 😊 I also noted that already about 8 scenes in the intro have been broadcast in these first 8 episode so that is cool. One scene from the intro seems to be where the ML has probably been reborn since he is dressed like a normal person rather than the demon lord.
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You can try to look at it as someone choosing to donate their organs after their death and his grandfather was the best match. It may seem like a stretch but that is the situation presented here.