Okay, I'm going to give this my best shot. Please note that I watched, and agreed with, Gaijin Goomba's take on the original video, so that colors my take on this.
I do not know if this is a prequel or a sequel. If I had to bet, I'd say prequel, but here's what I got out of it.
The first girl isn't real. She is just an idea, probably your typical anime/manga sweetheart girl. Notice how she's wearing regular clothes i.e. a long sleeved sweeter, collar shirt, a mini-skirt done in sailor style, and those lovely thigh-high socks. This is the outfit that so many girls in manga and anime are always wearing. Because of this, she doesn't really seem to stand out (perhaps it's a way of saying the manga she's in isn't selling wildly), but in the back of her mind, there is this hyper-sexual version of her. That's the version we see singing, and the reason she exists is because of what happens to ALL anime girls: fans turn them into sexual objects. You know it's true. No matter the anime or manga, there are fans our there that want the girls to be in a swimsuit, underwear, or nothing at all.
Moving on. At first, she fights that temptation, or it doesn't bother her. But notice that things start to change when she takes a bite of the apple--and I'm sure most of us know what a girl taking a bite of an apple symbolizes. Anyway, she starts to change. Suddenly, she has a wand. A wand that looks a lot like those magic wands that magical girls always have. I take it that this is her, or at least the idea of her, trying to get more appeal (perhaps the author of the manga changing things, adding a touch of magic in hopes that more guys will pick up the work). Also note that she has a cat, another cliche in magical girl stuff.
Anyway, once she gets her wand, we see the sexual image of her again, probably saying "Hey, I'm still here, but if you think this will work, go right ahead..." Thus, she creates a whole world with her wand. She made up all this stuff, trying to make herself stand out (perhaps the author making the world to show its appeal).
What she does next is create her companion, which if I had to guess, is usually the boy character in these animes and mangas. Notice how it doesn't really have form, personality, or...anything. That's true of a lot of guys in these types of stories. They are presented almost as blank slates so that the reader can put himself in their shoes. That's supposed to be the reader she's leaning on.
Anyway, she looks over and sees the sexual image again, but ignores it and decides to wait. And for a moment, it seems the waiting will pay off. Her comes her knight in with his horse. Her story is finally doing well, and she's getting fans. Or, it could be her imagining what will happen, I'm not sure.
Whatever it was, it doesn't work. Her world fails and no one is interested in her (perhaps this is the author being told that no company wants to run the story because no one will want to read it). Enter the sexual version again. Notice how it pans down her barely-covered body, and then it stops...where it stops. Note how she has her fingers aligned. That...is a very suggestive pose for a girl, and I'm sure the next shot of it cutting to her face and her seeming to do "stuff" is not an accident.
This is her giving up. "Fine. I'll be hyper sexual, just like all the other girls!" (Fine, I'll make it a fan service story, and then I'll sell it no problem.) Her regular clothes disappear and she enters the real world. My take is that now the story she is in is published and it's out there. And what does she do once she's there? She DESTROYS the world. She turns girls into flowers, something that is meant to be seen as beautiful and nothing more, symbolizing how guys that get sucked into these things view all girls as nothing but pretty objects. Then, we see a guy get stripped down to nothing, and he starts tearing his chest out in the shape of a heart, but then he crumbles. I take this to mean that girls like this fill an imaginary hole in guys. They think they're in love and they want a girl exactly like this girl from the story, but she can't fill that hole and they fall apart.
She ends up creating her whole world in the real world, destroying a lot of lives (I think it means she's done this to a lot of guys), but then along comes the guy and his girlfriend.
The girlfriend, the real girl, is everything this fake girl is, plus more. She's cute, she's devoted to her boyfriend, but she has one thing the fake girl doesn't: She's REAL. She can really love her boyfriend, she can actually be there for him.
This destroys the fake girl and her world. She realizes that she'll never be like that, and she flees, heartbroken. Her spell won't work on this guy she thinks, and in that brief moment, she sees everything she'll never have.
We now see her stripped naked, crying. She's showing everything, using all of her tricks now, and it doesn't work. Or does it? We see a shadow appear over her, and suddenly this girl has emotion on her face.
What I think happened is that the boyfriend went, "Hm...this looks interesting," and the girlfriend, feeling perfectly secure in her relationship, said, "Good ahead, sweetie". Thus, the shadow we see at the end is the boyfriend starting down the dark path that he ends up on in the first video.
Anyway, that's my take on it. Again, I must remind people that I agreed with Gaigin Goomba's take on the first video, so that makes me see this video in a certain way. I could totally be wrong on everything here for all I know. I also am well aware that I don't speak Japanese and thus the lyrics could paint a very different picture.