It IS true, her own memories which she created for herself during the time she was independent as well as her sense of self and mind belong to her, not to Sora, the memories her physical existence was based on belong to Sora, and since those had to be given back to Sora it caused everyone to forget her.
Actually, everyone forgot her because even those 'independent' memories were simply offshoots growing from the memories that belonged to Sora. As Namine put it, 'everything about you was built on those memories'. That's why everyone had to forget her; giving back the base memories of Sora's meant everything else she'd added to them had to go too, severing all the chains of memories.
It's kind of like the whole thing in Back to the Future where Marty started fading out of existence because he stopped his parents getting together, because all events leading to his birth were built on that base moment. And trying to say otherwise is basically trying to ignore the whole point of the whole 'tragically inevitable death' plot Xion had going on.
I thought the whole point of Xion's story was her coming to terms with how she was doomed, and that there was no way to save even a memory of her. It was about someone with nothing to call their own trying to find a way to make at least one good impact on the world before they disappeared, through friendship.
The memories others have of Xion are sealed away, she however still has her own, and her mind and sense of self are just sleeping inside Sora, just like Roxas, only a bit deeper and more hidden.
It's the same with Naminé in Kairi concerning their current state.
First of all, rather than 'seals', you'd be better off talking about how the links of memories are separated from the chains, which is what memories are in this series.
Second of all, by saying that Xion's existence is the same as Roxas and Namine is, again, ignoring some of the most basic parts of their plots.
Roxas and Xion were both born from something of Sora's, yes. Roxas was born from Sora's body, and Xion from his memories. However, thanks to Kairi, Sora didn't need anything back from Roxas until Namine messed with his memories, and all the separated links started flowing into him. That is the only real reason Roxas HAD to go back to Sora at the start of KH2; to return the memories that had leaked out, that Namine needed so she could fix the chains and wake Sora up. Despite having these memories inside him, Roxas was a Nobody and didn't need them to have his own mind. Because of this, it's reasonable to think Roxas could be sleeping inside Sora somewhere. This is also supported in the FM+ scene with Axel; he's definitely sleeping somewhere.
On the other hand, Xion was a Replica, and everything about her were built off memories belonging to Sora. She could not have developed a mind of her own without them, unlike a Nobody. And although it's absolutely true that thanks to unique experiences she grew her own persona, it is an important point in Days that she can't function, exist or even be remembered without those integral base memories. Which is why it's difficult on a logic level to imagine her coming back, or even how she sleeps. How will they bring her back, if she needs memories that Sora also needs? How can she be 'sleeping' as she is, if pieces of her are out working in their proper places inside Sora? The way chains of memories work means they can't exist at the same time.
I have no doubt that they will bring her back, but I have huge doubts that it will make any sense based on the information about memories, chains and replicas that we have been given so far. I imagine we're supposed to swallow it in the same way they expected us to swallow Xion somehow speaking to Riku through the Oblivion keyblade from beyond the grave.