About the restructuration, well, this is quite complicated so what I propose is to put as Chapter 0 from page 1 to 10 (of Chapter 0) and then put the rest as a single chapter starting page 11 and 12 (
since in Twitter, the artist started putting number from this one).
And any bonus like page 18 to 20 to be renamed as chapter 6.5 (because on Pixiv, they're placed before chapter 7).
Same deal with futur one when the artist decide to bundle other pages in his Pixiv account which might have other bonus.
EDIT: Ah, also, I don't think there's a need to put the volume number too since it's not serialized as a book yet (if the artist ever does).