Okay, so Ayaharu's setup for this one actually confused me a bit, but here's the gist of it. This is set four years after the incident in the bamboo forest. The idea is that Mizuchi was 15 by then, and the previous miko didn't have a kid yet.
The top-side panels are imaginations – the what if she's still alive. Four years later, Mizuchi would have been an adult, and would have been there for the scenario above.
With that being said... you reckon there had been intermarriages between the Hakurei clan and the Miyadeguchi clan in the past? Like, if we assume such a genealogical line on the tree, then Reimu and Mizuchi would really be related by blood.
