Handcrafting an ebook from scratch is most admirable but also very error prone. Why waste time on creating an .ncx file, when Sigil
will create one for you automatically? (Sigil creates .epub files, however, KindlePreviewer will compile them.)
I'd recommend using Mobipocket Creator
for your first steps. If you had compiled your project with Mobipocket Creator, you'd have gotten 2 warnings:
One about an unclosed node in line 23 of the .opf file and one about an unclosed node in line 14 of the .ncx.
Both files are syntactically incorrect. (The error in the .opf is actually in line 17.)
This wouldn't have happened if you had used Sigil, because it makes sure that all file are at least syntactically correct.