Thank you for the explanation; you've opened my eyes. You proposed some really interesting ideas there. But in that case it really seems like Seishin has really come to depend on Sunako emotionally; when he has problems, for example in this -- when Seishin and Toshio finally clash and it seems like there's no way back, Sunako is the one he seeks out for. Perhaps that scene could be translated as this too.

Yes, that I do agree -- it seems like in a way, Toshio is his anchor. Or was. And now that he's finally lost that anchor, he finds the replacement in Sunako. Perhaps that's why he goes to such great lengths to rescue her, even denying her wish to just burn to death. Is it just because of Tatsumi's wish? Probably not. Seishin is detached, but it seems like he's not very 'independent' emotionally as a person in this case? People who find their purpose through other people, I think Seishin is sort of like this too, well, dunno. Perhaps he's a person who ultimately wish for someone or a group of people he can connect with, can get close with. But for everyone, when they've managed to find people who can understand them, logically they wouldn't let go of them so easily.
And that Seishin has finally found answers to several things; yes, most of them are thanks to Sunako. One thing Sunako got wrong about him is apparently his idealism. (one thing Seishin happens to be most sure of) But actually, I think he's actually got the glimpses of answers -- but those are like puzzle pieces, a jumbled mess. Sunako helps him arrange them, forming a clear picture. Even if perhaps those answers have always been there and he kind of notices that, he cannot ultimately get them until Sunako helps pointing those out to his face.

All in all, I'm happy that Seishin has finally untied himself from the village, even if he doesn't find the price thrilling at all. But nah, poor Sunako. She'd probably be happier in death... Dunno.

When we put it that way, it really seems that Sunako is a better companion for Seishin, haha.