I think the other chapters and first six ones being not at the same level or in different parent(except in Part I, Part II) is confusing, not good design.
(There are too much such bad examples in books from gutenberg.org)
iBooks doesn't support tree node control like ADE, all nodes are listed expanded, different levels being expressed with indent. So heavy nested TOC will be shown as a large list.
Further more, long TOC list and complex TOC tree will make it difficult for users to browse on iPad and other eReaders.
A simple TOC list within 30 items is better, if necessary, a simple two level TOC tree is enough.
BTW, this is also the reason why ePub Maker still doesn't support TOC tree.