I may have discovered what was causing this. I use Ghostery to block a lot of ads and things of that nature, a few of which I unblocked for testing a website. I blocked these again and the box seems to be gone for good... hopefully.
There's really nothing you can do here. The views under "All Messages" are hard-sorted the way they are. Even if you try to change that in index.ini in the mail folder, Opera will revert your changes on you.
IMAP folder views and label views etc. are sorted alphabetically (simple ascii comparison) in ascending order.
For labels though, you can prefix their names with an "_" for example to get them to show up above the "a" labels for example.
The shortcut is Control–Tab and Control–Shift–Tab.
By enabling the Advanced keyboard shortcut setting, you can use 1 and 2.
In the upcoming Opera 12, you can use Command–Option–Left and Right as well.
You can achieve the specific shortcut you wanted by modifying the Preferences file. This is unsupported and I will thus not provide exact steps. There are other examples in the forums and elsewhere on the Web for doing things like this.