That's pretty much subjective because Xion still has detractors. She's not all that ahead of other girls like Kairi and Naminè. No one can argue statistics and claim one thing or the other because there's no way to fully measure opinions of the entire fanbase. The only one we know for certain that hit success is Aqua, and it's due to the mass support and consistency in that support. A sample size isn't representative of all.
And what I truly meant is how Xion (and Days to an extent) reeks of every thing those self-insert 14th member fanfics have been known for. And Xion is not even a subversion like how Naminè was. She plays it straight. The "suffering" and "tragedy" is only used to elevate her in sympathy and spur on the emotion of the main male character. That, and 358/2 Days had the plot dip in to fit Xion as an insertion. Which I've talked about in the thread proper and not going to bring it here.
I'm not sure on her quality of writing or what her experience entails, I'm just going by the fact that she inputs in much the same way Shiro Amano does, her personal preference within the characters and the novels. Hence, the ascended fanfiction writer. She definitely has a penchant for SoRiku, for example, lol.
Y'know, I've kinda gotten sick of the "fanfic" criticism in regards to Xion. If I've learned anything from the Internet or pop culture in general, it's that you see certain themes and motifs pop up again and again, and what makes a Mary Sue a Mary Sue is going to change depending on the person. As a recommendation, check out Overly Sarcastic Productions and their video on the subject on YouTube, it does a good job illustrating what I mean.
To look at some of your points, let's take the character aspect of Xion out of the picture for a sec, what do we have? A replica designed to cipher Sora's powers. They essentially built on what was established in CoM with the Repliku. And while it does seem odd at first to have a 14th member in an Organization named for having 13 members, given what we know about Xehanort and his backup plans upon backup plans, it makes sense for Xemnas to keep Xion around as such a backup in case Roxas either defected, was destroyed or deemed inadequate.
And it wasn't just because of Xion that Roxas left, she just got the ball rolling. He left because he was tired of being left in the dark, especially from Axel, his best friend. Again, Xion may have been a factor, but it was ultimately due to lack of trust from the Organization as a whole.
As for Kanemaki's writing having preference for certain characters, while I have yet to read the KH novels myself, couldn't the same be said of all writers to an extent?
I won't argue that there are detractors to characters like Xion, and like you said, it'd be impossible to measure opinions of the entire fandom, but just because Xion was created to play the role of tragic figure in the story doesn't mean it was done badly. You can have a trope be played straight and still have it be compelling. Were there aspects of Xion's story that could with some fine tuning? Sure, I'll agree to that, but please, let's just drop the fanfic comparisons.
