Hmm, no, I can't say I completely agree with it.
Before she met Shiori, Sei was in what could be a suicidal state of mind, at least, to a degree that she was sick and tired of everything so much that she accepted being as she is and left as she is. But, I suppose, like in the train station, there was this little hope in her that someone will save her.
But obviously, it wouldn't just be anyone because before that she was surrounded by people who care deeply for her. Yet, she didn't see them, she still thought of herself as being alone. Seeing Shiori was like divine intervention and for this angel to actually have the slightest interest in her suddenly makes life worth living.
So, Sei was a victim of circumstance. It turned out that the one person who became her life was to dedicate her own life to God. I agree with the doujinshi about Shiori having God to fall back on when Sei is gone and how Sei has absolutely nothing when Shiori leaves her.
But it's unfair for both of them because Shiori had this set plan to become a nun but then Sei comes along and suddenly, her conviction is rattled. It would have been better had she mentioned it the moment she and Sei met, but she's only human and couldn't help but indulge herself in that bliss.
True, being so absorbed in someone makes you happy and lonely at the same time. But the loneliness is just a price to pay for that great amount of happiness. It's just like a drug. Sei was willing to live with this irony but Shiori wasn't because she had God to keep her from becoming lonely. But I don't think it's Sei's fault, nor is it Shiori's. They just dug themselves deep beyond control and that would have been fine, only Shiori wanted to crawl out of that hole while Sei would rather stay there and bask in Shiori's warmth.
I think Yumi, with that analogy of hers, meant that at that time, the world was cold and bleak and Shiori was the world to Sei in a sense that she was the only source of warmth, the only one willing to shelter her. Maybe how we perceive the world change like the seasons though- with winter being the harshest. Maybe they met during the winter and sought refuge in each other but as we all know, winter will pass and spring will come.
To me, what Yumi said about saying "There's no other choice" with a laugh, in regards to Sei asking if one of the two people hiding in that cave refused to go outside, meant that she would just look outside and, no matter how much she wanted to go outside, she would still stay with that someone in that cave until they were ready to go outside. Until then, she'd be contented with just looking outside.
Of course, if this is what she meant, it'd be wrong about Sei and Shiori.
I agree that Sei keeps to herself emotions outside of light-hearted and jests, at least, to people she met after Shiori. In Shimako's case, I believe, she was still recovering then and was still acting like how she did when she was in 2nd-year.
Youko she has known before and after Shiori and maybe that's enough for her to trust Youko with her anger and fear. It's sweet, yes, but there's the risk of getting slapped ^__^ And when she does sulk, it's usually alone- except when she mentioned Shiori's name in front of Yumi and Yoshino. But maybe she doesn't always hinder herself from expressing how she really feels because she's very genuine (or a really good liar); maybe she just doesn't indulge herself anymore.
This is long, I know (and you know that means I didn't check back to edit). I'm sorry for rambling.