That's if that was Roxas' intention in the first place. In KHII, we got that he left the Organization to find out why the Keyblade chose him, and therefore find out more on Sora. He could have been, and was, completely unaware of what that meant concerning his existence. The role of him absorbing the memories of Sora, being his other half, could have done so many things in terms of slow revelations the more he kept getting them (or experiencing things through it, like "snarl of memories"). That should be enough reason to kick him into gear and begin his self discovery. They could have dragged it until probably Ventus' memories mixed in. Instead, to justify Xion being inserted and for her to have a purpose at all, this was given to her. Roxas himself sat around until the last few moments because his role got diminished to make room for someone else. The fact that they even needed to do this shows how forced the circumstances were to input Xion there. To add salt to the wound, she was a repeated development of what he did in KHII, only more expanded and more dramatic.
Diz would not have waited an entire year, whatsoever. It's out of character. It would have made a lot more sense if the Organization was a lot more involved, getting in his way, but they weren't. Riku is a mixed bag. He was so understanding with Xion for like months, and then suddenly, he wants his friend back as a reason for why he needs to knock out Roxas and get him to Diz (who is only now acting upon things despite knowing the situation a half a year earlier...). Don't even get me started on Saïx's characterization and this obsessive, personal vendetta towards Xion for no good damn reason other than needing someone in the game to be horrible to her so they can amp up the "poor Xion" spiel of how much of a tragedy she is.
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/Riku—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.