Yes Google fonts will work as they are not provided by the Ubooquity server but directly by the Google servers.

I have included the modification to allow fonts in theme in the next version (which is a few days away). I even allowed "eot" fonts, although I stringly discourage their use as they are only supported by IE.

Can't say exactly when the next version will be release (I'll try to relase a minor one in a week or two), but it will include a new attribute on entries that contain the current folder content:

<entry>
    <title>Files in current folder (34)</title>
    <id>293-files</id>
    <updated>2015-03-19T21:52:59+01:00</updated>
    <content type="html"></content>
    <link type="application/atom+xml; profile=opds-catalog; kind=acquisition" rel="subsection"
href="/opds-comics/293/?displayFiles=true" pse:current-folder-content="true"/>
</entry>

Let me know if you want something different.

Like I said in my email, I did not find anything in the logs. Sorry.

I quickly tried Challenger Viewer in folder mode but I was not able to reproduce the problem.
Anyway I would not have been able to do anything as I am not the author of this viewer. You should contact him directly.