I can't really comprehend your argument that Roxas' development wouldn't have made sense by KH2's time or what restraint it placed on Roxas' own story of the past. There were no holes either to fill in. The only mystery about his time in the Organization that wasn't mentioned throughout KH2 was how the friendship between him and Axel failed, along with how he could dual wield. An entire game's worth of content didn't have to be created for that alone. It's funny that you talk about the time spent for an entire year, but 90% of the game is three people eating ice cream on a clock tower. Nothing plot worthy occurred until near the end of the game. And how convenient, that Roxas only took action near that end of the game while he could have been going around discovering his own self instead of sitting by. That's a fault of Xion being included in. What other role could her insertion take otherwise? Nothing about KH2's prologue set up with him inhibited or mandated anything.Honestly, if Days was literally just about Roxas' time with the Organization, along with his friendship with Axel and struggle against the advances of Diz/Rikuâ€”that'd been fine.
But, all the problems you listed wouldn't have ultimately been solved this way. At a certain point, the lagging sense of "time" from Roxas' conception to his defection from the Org/capture by Riku is justified (or made convenient) by Xion's erasure. At the core, I believe that was her true purpose in this story, to nullify that sense of time. A long term battle of wits/power between the Org and Diz/Riku/Namine for solely Roxas sounds interesting, but with how KH2 played out, it doesn't coincide so well with that implication. All the above issues aren't a reflection of Xion's character narrative in Days, but rather story aspects that have actually been an issue since KH2.
Xion's story could be thought of as a means of justification for the lag of Roxas' existential development within that year time span. Meaning, her insertion wasn't the means to take away his development, but rather, to explain why it didn't happen fully for Roxas until the events closer to KH2. I would agree wholeheartedly that without Xion, Diz/Riku/Namine wouldn't have waited a year to get Roxas, and Roxas having the time for development in learning and accepting of his existence through Sora wouldn't be so impeded on. But, it already was, before Xion was created, i.e. this was already set by his development and events from KH2.
The main meaning of Roxas' story was meant to be told in KH2, with us knowing there being a large amount of time where what he got out of KH2 didn't happen yet. To justify this, they had Xion be the reason for this case, a misdirection of objective for those involved in this plot. She was something to deal with, a story to be told that allowed reasoning as to why that year passed the way it did. Roxas being the way he was in KH2 wouldn't have connected well with that amount of development he would've got in a year's time. Depending on how they'd deal with that sense of "time", it would've functioned more as a retcon rather than "prequel-like", filled in narrative. The alternative sounds great, but that type of narrative to the conflict over Roxas alone should've been noted more in KH2 to fully integrate itself in the story. It's no wonder Xion's involvement didn't really change Roxas's, or anyones, perspective in KH2. It would seem, the KH Team just cared about inserting more to the context of Roxas's time in Org without changing how his experience in KH2 was perceived.
Again, this is really more of a hole the writers already dug themselves in, and they used Xion to fill it. Her character, without considering this, still held her own to not simply be reduced to just being "unnecessary" in this regard. She had a role to play, even if it wasn't performed at optimal value.
As for Riku's representation, I'm not really sure I agree either. In terms of who and what Xion was, this was something even he had to figure out, and even he still talked down to Xion when initially encountering her. He had more time to gain an understanding and a role to play in Xion's journey, something he didn't get with Roxas. This, again, is only due to what KH2 already set in stone for the time frame, this isn't a consequence of Xion as a character. I'd find it real silly for Riku to have this process with Roxas for a year, only to reverse it to how he was during KH2 events towards Roxas without any accountancy at all of it.
For Riku's overall story, yes, I agree that could've been emphasized so much more. But, neither Roxas or Xion would be needed for that anyway considering his own battle with the darkness.
Believe me, I can understand being disappointed in how they handled Roxas' story in this year time. Less is more doesn't always apply. Lol But, the flaws surrounding this plot of Days/KH2 isn't a direct result of Xion's character. Even if her character was created as a justification, her narrative showed us more than that through her own journey and friendship with Axel and Roxas. That has its own value.
Overall, my stance is just that the flaws that this facet of KH's storytelling retained was a problem set by KH2, not Xion in her entirety. She may have been used as a means to try and resolve these holes, but I don't believe her character largely suffered from this, nor did any other character as much as this thread implies. It's not perfect, but there are good things that surfaced. This year's time could've very well been different without Xion, but if so, the difference should've effectively been made precedent in KH2. I don't blame her, I blame the writing set in KH2. If as a result people dislike her, I can see why you would by association. If her character doesn't stick for you, that is what it is. I would just implore you to see it as being another victim of this game's flaws like every other character, rather than her being the source of Days/KH2 issues.
At the end of the day, Xion and Roxas do have different personalities and approaches in how they dealt with their existence in the world. They correlate, but they're not completely the same. As far as I'm concerned, Roxas story, his journey in that regard, was meant to be told through the time and place relative to KH2. His role in Days, Xion didn't necessarily "steal" this away from him, the writing already mandated for this to be in KH2. Of course, there are many things that could've been better for every character, including her. Erasing her completely from the story wouldn't directly fix these issues created before the game's conception. Roxas and others lack of development during this phase isn't a consequence of her, but the writing.
Personally, I just don't believe calling her worthless or unnecessary is really fair in this regard. -shrug- I don't know....
Disregarding that, there are good things to come from Xion's character, mainly through Axel and Roxas. Those personal journals (for every character) were pretty intriguing and added some depth that matches even the novel's character introspection. I found it to be quite enjoyable.
KH2 began with Roxas being captured by Riku on the orders of Diz, situated in a data Twilight Town so not only can he live in blissful ignorance (which didn't happen because NaminÃ© came in like a wrecking ball), but for him to be safe from the Organization members who may look for him. That facet of Diz's reasoning would have made a lot more sense if the Organization was already a thorn on his side throughout the year before, and would have made it have more depth in a more fulfilling manner. Roxas defected because he needed to find why the Keyblade chose him and in turn, Sora was the answer. That does not mean he already had chosen to return to him the same way Xion had done, it did not mean he figured out that his existence was in jeopardy. Just that he wanted to find the truth behind his origins, his ability, and the sole way to do so was to seek answer because his friend wasn't budging. All Days had to do was accumulate what led him to that choice, which again, would have fitted if he was the active member going through the motions and investigating. There was nothing Xion did that Roxas would not have been able to do on his own, including the memory absorbing since Xion was taking it away from him in the first place. On top of that, having him and Axel as friends without a third party not only would have separated the shoved down your throat trio concept, it would have differed Roxas from Sora by having an unique connection that isn't a rehashed SoRiKai, further implementing that he is his own person. Then, if you add a little Riku side story, we more than likely would have had a more sensible game without the plot caving itself to keep Xion alive for a year or make her so "important". It was so try hard, goodness. It's like any chance that Xion could be forced somewhere, they took it. I could not stomach it. And with KH, I've tolerated a lot of bull. It doesn't bode well coming in and having to accept this additional person feeling like it's forced.
Days as a whole was unnecessary (going back to the first point made in the first paragraph). We all know the reason for its existence is due to the crazy popularity of Organization XIII (oh, wait, XIV, oh but not really, it's XIII, but there's fourteen, despite that NaminÃ© showed that she was able to live and interact with the members without actually being one), Axel, and most of all, Roxas. And it didn't even accomplish the potential promise it held, making a new complete character bulldoze the entire narrative, leaving the members as very much side pieces, rendering Roxas to lounge for majority of the game, and having the story revolve around angst. KH1, CoM, and KH2 worked without Days. Did it expand on Roxas' character? Certainly. Was it needed? Not exactly.
Overall, for people to bypass the flaw of Days, along with the faults of the narrative created by a character that was added in (Xion), you would have to care for Xion as a character and get attached to the sob fest. It's practically a given. If that didn't work, because it was more than likely their intention, for the audience to pity or sympathize, then the fans are not going to ignore Days' fallacies.