Hey, I've just had a quick look at this project and it certainly looks interesting. I think using parent items instead of using parent-title etc. is a good choice.
One thing to consider though: I suggest making publishers an object with attributes: name and location. Because there are rare cases where a book is being published by two publishers that each gave their own location.
You will want to render this as:
Location A: Publisher A; Location B: Publisher B

We've had some discussions about this over at the CSL repo, and a similar change is planned, but it's much easier to make such a design decision when there's still not much technical debt...