Responses
Something to append: It did take more than a half hour of dissecting the last few chapters to work this theory out, so while I feel like I may have come off as condescending, I absolutely understand and do not judge if you were confused by the ending and that's why you didn't like it. I was too.
While the ending was very low-key, it did seem like a happy ending, and I don't think it was badly written or even necessarily rushed. Based on both Hwayeon and Eunha's words, it does seem that the curse that was affecting Eunha's memory IS fixed. That's been a key element of the story, and the resolution of it actually makes a pretty decent place to end off.
I also don't think that what Eunha drank was Siyeon's soul or anything weird like that; think about it, just before it jumps to her retrieving Siyeon, she gets up and implies going to a physical location (presumably to Siyeon in the physical world), not her own mind. There's also the glow of a magical doorway behind her, which along with what she says sure seems to imply that however Eunha entered that void space, she was there to get Siyeon out. If Siyeon had fused souls with Hwayeon or Eunha there wouldn't be any reason to try to retrieve her because it wouldn't be possible.
The way they describe the space with uncertainty and the fact that a person is not always present within what is supposed to be their own mind says to me that whatever that space is, it's NOT Hwayeon's mind, but maybe something like a private dream realm that one projects their soul into. My interpretation is that there's only the risk of soul fusion if two people are in the space together for an hour. Since Hwayeon left just before Siyeon's time ran out, that would leave Siyeon alone in that space, but just unconscious in the physical world like Hwayeon was before Siyeon entered.
I;m not sure, but I think Siyeon may have lied about the tears thing. If Yoobin was supposed to gather tears from the sleeping Hwayeon, why was there no evidence that Hwayeon had been crying when she was shown awake? I think that Siyeon chose to stay and isolate herself from Eunha so that Eunha and Yoobin could be happy (her smile before going in could have meant that she never planned to come back). Hwayeon explained this to Yoobin when she returned along with the details of the curse and the fact that the solution was already complete. Yoobin and Hwayeon respected her decision, but Eunha went to get her anyway.
I think that the ending is deliberately somewhat open and also chooses not to spoon-feed the audience everything, which is a style of storytelling that I'm sure some people don't like, but isn't bad writing. The ending of Citrus was bad writing. It is still possible there will still be a sequel series that explains more, but there will not be an epilogue. The Korean version of Lezhin's website has the series marked as completed and Chapter 55 was a month and a half ago.
A Very Late Edit: I personally like to think that Eunha, Siyeon, and Yoobin are poly following the ending. I don't know why I didn't make note of this when I originally posted this breakdown.