I agree that it's likely for Himawari to only exist as she is/at all due to someone else. I'm honestly behind on/unsure of the menagerie so I can't comment much.
Assuming she is the unborn, reset daughter of Ayane, the most supported theory/largest consensus from what I can tell, I only see three real branching paths from her being a baby during a reset;
That's probably the last ounce of Himawari speculation I got, it
kind of sums up everything I've posted about her. I almost doubt we'll get any answers as to how she exists at all. I think what she does and what happens to her will be vastly more important, and for that we probably need to wait a decent while. I'd hate this bitch if she wasn't kind of cool.