I've had this one for awhile, just wasn't sure how to put it. It started when I thought that Miku and Mafuyu symbolised the two halves of Kirie; an older self bound by destiny, and a younger self doomed to the fate of their other half. Reading more into the symbolism, I came to the conclusion that Little Kirie might not have actually merged with her adult form, rather crossing over as her own independent spirit, free from the duty of being a maiden. This is mirrored in the Hinasakis, as Miku escapes without her sixth sense and Mafuyu takes on their burden.
From there, I built a bit on some of the thoughts I had before. Miku's equivalent of Kirie's duty is her sixth sense; both are inherited from lines of women cursed in some way, the Afterlife at its root. As mirrors, they're both split in two. In the end, I think the story is genuinely about Miku's destiny, as Mafuyu shares it with her, which was actually represented in one of the games' wallpapers. His sacrifice is for Kirie, but going with the metaphor, you could see it as also for Miku as well. Had Mafuyu not been Miku's priority, if she'd been in the mansion alone, would she still have left in the ending? I think it's possible she might have stayed behind out of understanding, too, which could give another purpose for her sixth sense: Miku was always supposed to be in the mansion, she was always going to be at the Hell Gate. Could it have disappeared because Mafuyu took over for her, like Yuu taking Rei's tattoos, or, like Kirie, he's acting as a Pillar, but for Miku? Or, an even stranger suggestion, would sealing the Hell Gate have an effect on the living who can see the Other Side? Food for thought.
Also, doing some digging, I noticed a bit of wording in the guidebook I thought was really interesting regarding the Calamity:
Since the Ceremony of the Ropes was performed using laceration rope created from a shrine maiden who still had worldly connections, the ritual failed and the gates opened slightly, allowing the Malice to leak out.
As it's true, the doors didn't open all the way, though all the records indicated they would. The conclusion I came to was that similar to Ouse in 5, though Kirie's ritual was a failure, she'd still successfully become a Pillar and was able to prevent the Calamity from reaching its full scope, though bearing the full brunt of the Malice as a result. It was also strange to me the 5 mirrors located around the mansion would break as a "warning" of the broken seal of the true mirror, which happened well over 100 years ago. Instead, I think it was a warning, foretelling Kirie's end as a Pillar, as consumed by the Malice, she was unfit to seal the Gate for much longer.
The result of the broken mirrors indirectly lead to the Hinasakis entering the mansion, bringing us back to the beginning.