You can try adding opera:config back and access most preferences that way.
You can make Opera Mail the default.
Links in Opera Mail will open up in the default browser on the system. So, set your default browser in the system. If you want Opera Mail to open links in a specific browser, I'm not sure you can do that. What you'd do though is configure the http and https protocols in "alt + p -> advanced -> programs" in Opera 12, close it down and look at its operaprefs.ini in its preferences folder. There should be http and https entries in there that you can copy to Opera Mail's operaprefs.ini. But, don't know if it'll override Opera Mail's default handling as expected. Also note that in Opera 12, the https and http settings might be stored in handlers.ini where operaprefs.ini has a protocol version section that tells Opera to use handlers.ini. You might have to duplicate both parts to make it work in Opera Mail. But, again, it might not override Opera Mail's default handling as that might be hardcoded.