Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.
The initial meeting scene plays out almost identically to Mikazuki's dream from the beginning of the game. This would imply that Mikazuki's original dream predicted the future, which he hasn't been shown to be able to do, that the Touken Danshi just happened to act out his dream in real life, or that it was meant to be a hint that the entire cutscene was happening in his head again.
There's also the fact that the ending appears to be even more dreamlike than Mikazuki's actual dream, which seemed like real life until The Reveal of him waking up. There's a dreamlike fog coursing around that no one notices, and the scene as a whole has a dreamy look to it, which isn't helped by the painterly border around it.
Omokage's comment on the background music, I think, is the biggest "it's a dream" tell in the ending. Mikazuki's music box is nowhere to be seen, so he can't possibly be talking about the song from it, but the background composition sounds similar to the melody he said he wanted to hear, which explains his reaction.
Finally, this is a personal thing with no evidence, but I don't think the developers would undermine or undo Omokage's sacrifice by having him magically find a way out of his dream prison offscreen after he made a big show of how he can't physically leave that place with them. It would make more sense to me if he discovered a way to use dream abilities to connect with the others through their dreams, which is a power Kurokage possessed in the first place, I believe, but was otherwise still trapped in that endless void. Either that, or he simply created a new dream reality for them all to meet infrequently, which seems less likely since it's still too similar to Mikazuki's dream. But that could have just been an oversight on the developers part.
Whether the Saniwa exists or not, what they represent for the Touken Danshi, and how it relates to the games overarching themes about hope, persisting in the face of adversity, living without regrets, letting go of the past, and the importance of bonds, is more important than who they are (or may have been) as an individual.
To begin, we learn very little about this Honmaru's Saniwa despite their importance to the Touken Danshi - not their name, gender, or even much about their personality other than that they were kind and probably wise. While hiding the Saniwa's identity from the audience used to be common in the series, more recent spin-offs have been more daring in giving their versions of the Saniwa distinct identities. Keeping this in mind, Musou could have gotten away with establishing an much clearer identity for the Saniwa, as they aren't a stand-in for the player in this game, but rather a person that exists (or existed) independently of them. With that in mind, it feels like it was left out for a specific story-telling purpose, if not to not take away from the stories of Touken Danshi and their journey.
It’s also notable that the disappearance of the Saniwa isn't nearly as plot relevant as most of the other events in the story, as getting them back is primarily just there to serve as a driving motivation for the Touken Danshi. It's the reward they'll receive at the end of their journey for believing in their bonds, growing together, and persevering against the struggle. And when the time comes for the Touken Danshi to receive the reward, the players don't even get to see it delivered upon, or if it was delivered upon, in either ending because it ultimately doesn't matter as much as the journey itself and the symbolic meaning the idea of the Saniwa served during it.
Mikazuki's music box being a representation of the Saniwa also lends to this theory, as he says the melody it plays reminds him to believe in his bond with their Saniwa, later referring to the Saniwa themselves as "the melody" of Honmaru that the HRA attacked them for. This same melody grows on Omokage when he allows himself trust and bond with the others, despite his lack of a personal attachment to their Saniwa. He never even meets them, as far as we see, and in games second ending, which can only be unlocked by maxing the bonds between him and every Touken Danshi, the Saniwa is still notably absent, but the melody that represents the bonds created persists, and he is shown openly appreciate it its beauty when he hears it.
