Use
epubtweak or Sigil if you want to poke around inside epubs. I've had mimetype problems with 7zip in the past (because of reordering of the files), though WinRAR has worked fine.
Alex,
this post describes how to change fonts using external css references. You still need to modify the css, but don't need to embed the font in each epub. If you want to alter the default fonts you'll need to wait until PRS+ or a similar hack is ported over to the new models.