I got some recommendations privately:
You have to re-arrange the playorder attribute in the toc.ncx file, it should be always in a ascending order. And for the I/O reading error, you should removed the doctype declaration in the HTML files. Pls do this and run the epubcheck again.
I'm not sure all of them are clear enough... I'll see what that could mean
Meanwhile, you could use online ePub validation service
offered by ThreePress