Regarding the triangle, it is not so much that the fan girl is there, but the timing of the whole thing. Some of us spent three, almost four years waiting for the main couple to become a couple, we get the first kiss, the hype is at its maximum, and then the next chapter is the start of the fan girl arc. Hatred, or at the very least dislike, towards the fan girl and/or narrative choices of the author from some of the long-time readers was bound to happen.
Now, what I do not agree with, however, is that this plot is being forced by the author. When I think about it, it actually makes perfect sense. The fan girl was shown over the years constantly trying to spend time with SJ, and pretty much always getting turned down. If memory serves, the fan's response was always along the lines of "what, again", indicating this to be something of a trend. She could not have been promoted to full rival because she was unable to form any kind of meaningful connection with SJ, apart from being friendly with her during breaks in school.
SJ always carried herself in these situations in a way that made it obvious she barely even registers the girl, even as a friend. As a parallel, SJ did not just approach QT and confessed out of the blue, she first spent time with her, got to know her, and then confessed.
The only way the fan girl could have been made a rival was if she did pretty much the same thing she is doing now, only if she did it earlier in the story. That, however, would make far less sense. It was hinted, and this turn makes it obvious, that she has real feelings for SJ, it stands to reason she would try something more than just confessing randomly in a school hallway. She was trying, and failing, to get close to SJ, form some sort of connection before opening up about her feelings.
Now, right before this arc started, what did she witness? The birthday party scene. She realised, to her horror undoubtedly, that SJ already has someone she likes, and is, perhaps, even in a relationship with said person (since she had no way of knowing that only after said scene QT finally made the first big leap towards something more than friendship). Remember, the fan girl knew nothing about QT previously. SJ never said where she had to go and whom is she meeting when turning her down.
It actually makes perfect sense for that party scene to be the catalyst for the fan girl finally making a move, probably out of sheer desperation.
Also important to remember, all this takes place in our world, at least, we are not given any indication to the contrary. Therefore, it is also possible the fan girl never actually intended to confess, since Asian country and being gay and all that. It is also probable she had no idea SJ swings that way. If that is the case, her seeing SJ clearly enamoured with another girl is even more of a punch, and would make an even better explanation for this sudden turn of events, in my opinion.
I intended to say at the end that despite all this, I still hate the fan girl, but now I actually can not really bring myself to do that, lol. And they say you can not reason yourself out of emotional stances.