Question about Oasis(2002)

Just saw this harrowing, at times disturbing, at times beautiful film for the first time. My thoughts about the movie are very positive and a movie hasn't moved me like this in a while. There is just one aspect about the film that remained a mystery for me. It is the imaginatory scenes sprinkled here and there.

In the first one we see a bird flying around in the apartment and a voice hymning beautifully, but as Jong-du enters the house, the bird transforms into a reflection of the mirror and the hymning turns into the noise Gong-Ju makes. This is shot from her POV. Later on as Gong-ju shatters the mirror, the reflections of the mirror turn into butterflies, she is again interrupted by Jong-du.

Later on, in the subway we see her without the symptoms of cerebral palsy as she stands up and teases Jong-du like the other couple in the subway does. We get a very sudden cut into Jong-du's POV and we see she is in her actual state.

In the repair shop when Jong-du is talking in the phone she, without any symptoms, gets up and pushes Jong-du making him drop the phone and lose connection. Jong-du is annoyed at her but then he calms down and start laughing. They are interrupted by the delivery and in the next shot she has her symptoms again.

At one point, Jong-du tells about a dream he had in which he and Gong-ju were in a room with the characters from the oasis painting. A few scenes later we see this scene happening. At the beginning of the scene, she is in her actual state but then she is rid of her symptoms as they dance together.

After leaving Jong-Du's mothers party, Jong-Du and Gong-Ju go into a karaoke place. Jong-Du sings but when he asks her to sing, she doesn't. In the next scene they are in the subway station. They miss the subway and Gong-Ju loses her symptoms and starts singing, caresses Jong-Du and makes him sit on the wheelchair.

My question is, who is experiencing these imaginatory situations and what is the purpose of them? Jong-Du, Gong-Ju, both, or something else? The first one with the bird-reflection, I would imagine is happening in Gong-Ju's mind. It is clearly a POV-shot and her imaginations end when Jong-Du enters. The butterfly-reflection isn't shown from her perspective but she is alone in the room. Again, Jong-Du ringing the doorbell disturbs her.

In the subway it is Gong-Ju observing the couple suggesting that it is, again, her imagination. But the imagination ends very abruptly with Jong-Du's POV shot of her. As she starts laughing, we cut into Gong-Ju's POV of Jong-Du as he questions what she is laughing about. Maybe this is the moment where Gong-Ju's fantasy actually ends because after that, she stares with a longing in her eyes out the window.

I don't want to make this post too long by going over every scene again. But by writing my thoughts down and going over the scenes again, I'm more and more inclined to think that all of those scenes are happening in her mind. Further suggesting this would be that Jong-Du's behavior doesn't seem to chance in those scenes.

How do you see the purpose of those scenes? Is the first one just to show the viewers that she does in fact fantasize about things and the latter ones there for how she would like to be perceived by Jong-Du and others. With the singing scene does she feel bad for being unable to sing in the moment and makes up for it in her fantasies?

I would be tremendously interested in your interpretations!