I've fixed this — thanks so much for your willingness to help, though.
There were two problems.
First — and I think most importantly — the eBook broke because I wasn't calling my viewport size in the cover.xhtml file
<meta name="viewport" content="width=1200, height=1700"></meta>
I think that forgetting to do this on each and every page
of a fixed layout iBook totally breaks the entire publication.
Secondly, it looks like
is required for the cover item in the spine.
Of note: my cover image is now 1200x1700px, and it seems to work just dandy even though I've read that it should be less than 1000 pixels wide and high.
I thought that this would be a tough one to solve, but it was pretty basic! Contrary to what my art director thinks, turning off your computer and sleeping on a problem is much more efficient than getting frustrated and staring at code for ten hours straight