There is an interesting article about this here:
I notice it says:
Every file in your epub must be declared in the OPF manifest. This includes fonts. Put the font in the same place as your OPF file (usually a folder called OEBPS) and add it to the manifest. For example:
<item id="epub.embedded.font" href="MyFont.otf" media-type="font/opentype"/>
Maybe you need to add this to your code?
Sorry, I don't know about whether Courier New works. It's just what I extracted from the code that Calibre puts into the epub it generated.