I'm not sure why opera didn't connect. Opera should have tried to download the file. I've used Opera forever and never had problems downloading zip files.
From your description, the problem seems to be a server side issue. The server seems to be detecting your device as a mobile device and is rendering the ePub to a text page, then serving it to your browser.
The only work around I know is to trick the website into thinking it is serving up a page to a desktop client.
This can be done with a browser like dolphin HD. Change the user agent to either desktop/iPad and you should be in business.