One problem might be that you put a
div inside a paragraph. Epubcheck flags that as an error. Use a
span instead.
To make iBooks display embedded fonts correctly, you have to add this file to your book:
com.apple.ibooks.display-options.xml
I don't know Hyper at all, so can't give you any advice on that front. FB Reader is notorious for ignoring all formatting anyway.
And you'd better ask a mod to move this to the epub forum. The topic is not specific to the Calibre editor.
EDIT: I overlooked what you said about trying it with a span too. I'd still use the span, but if the div isn't the culprit, the display-options file should do the trick for iBooks.