Jupiter Shim

I dont think the dad trapped his mother or anything. To me, it seems that the mother started having/showing mental issues during the mariage or after having Jihwan, and the father at some point didn't allow her to go out so that she dosen't "Shame him". He seems like an important and rich man so it's possible that he just dosen't want to show to the World that his wife is crazy. To me it seems that Jihwan got the psycho part from his mother's side. His father is just your typical cold rich father in Korean dramas who wants everything to be perfect. As to why he dosen't like Jihwan, well, because he's crazy and does crazy and dangerous stuff. I mean the chauffeur/Bodyguard warned him not to pity Jihwan.

With this last chapter, it's really difficult to predict what's happening next...
So Jooin chose to let both of them go, which is pretty understandable for me at least.
With Yahwi it's clear that they both loved each other but Jooin got so much hurt in that relationship that he can't be with Yahwi no more. Or at least not now. Maybe they will meet in the future and fall in love again, in a healthy way. But for that to happen, they really need a long Time skip, like them meeting as 30 year olds or something. But it's as if it's going to be a completely different story. Don't know how they Can pull that.
As for Cain, it's more ambiguous in the sense that we're not sure of what Jooin feels for him. Jooin himself seems to be still confused about those feelings..we know that he's precious to him, that he finds him attractive. But does he love him? He says that treasuring is not loving but at the same time we see him in other chapters blushing with his Heart beating when Cain is close to him. Him saying that his feelings for Cain came drip by drip, and also him sleeping at his mom's place to not be alone when Cain left.
To me it seems that he chose to let Cain go because of two reasons. First because he doesn't want him to abandon everything(family, Friends, studies).
Jooin cares a lot for Cain and he certainly doesn't want to ruin cain's life. And secondly, because Jooin doesn't want to be Cain's master, that is not the kind of relationship he wants. Even in their last conversation, Cain still talks about Jooin as if he was the same person of the past life. With Cain's letter, Jooin maybe came to realize that Cain has this much love not for him but for his past life master and that makes their relationship vague and based on something that Jooin is not aware of. That makes Jooin doubt his own feelings too because he doesn't know much about the real Cain of this World. So there are for sure some unresolved issues between these. I think they will surely meet again in the future.