I liked how, in your review, you compared SubaHibi to that of an unsolved equation with a large amount of parts that comprised its whole. That is very true, and SubaHibi is indeed mainly recognized by a lot of people as a story which can only be freely interpreted in a subjective manner by the audience, since only Sca-ji really possesses the answer to the equation. However, from my own experience, I can't say I completely agree with that (which I'll explain why at the end of my post).
Of course, to what extent you look into this story depends on how much of an utter completionist/perfectionist you are. In theory, understanding what SubaHibi's plot 'is' would make it all the more clear what the meaning, the message, the thematic significance of the story is. With my current perception of the story, I wouldn't be able to bounce off my ideas on those things to others without first going into my idea of the equation's answer (especially if it's concerning Zakuro and Rabbit-Hole I).
Here are my answers to you on those points you bring up as "A" and "B" (I can't fit the other points about Zakuro and the Gondola Ride here, so...):
which is by default given to people when born. So in a sense, it's saying that people who are born but end up broken can still grab an opportunity to be reborn, for all we have is our internal world to allow it. And of course, SubaHibi also shows exactly what causes people to break themselves, in not accepting their existence, and consequently, their own birth into this world. If you look at someone who's mentally unstable and/or hospitalized, your exterior viewpoint (the initial impression) immediately sees a person who could never be happy. SubaHibi is trying to show the audience the inside of such a person to overturn that expectation.")
on the 向日葵の坂道 end that Yuki's existence is essentially the soul. As seen in RH1, Yuki is the 世界そのものの少女 and a lot of the narrative throughout the chapter seems to be implying that the world is shaped by her will and desire, then that is reflected onto another world. I think when Yuki becomes aware that she is the World Girl, that transitory world starts to contract as she would be nearing the decision of whether or not to be born into the world ahead. She decides the former in this case, desiring for RH1 to have been but a passing dream. Thus as a result of her desires, it all becomes irrelevant to the rest of the story ahead. RH1 is basically a giant metaphor that portrays the same message. But in addition, it can also be the importance of thinking that; life is a blessing beyond what appears to be a curse, so you want to be born and live in it.")
I really hope that I was able to present this in the least convoluted way possible. Though I would still recommend you take a second or perhaps third look at everything. (Especially
I think that SubaHibi's story, as an 'unsolved equation', most certainly has its sole answer embedded somewhere within the story. It's just in such a place that no one would ever think to look.