> Is Opus always better than Vorbis in practice, making Vorbis usage no longer recommended, or is Vorbis still useful beyond compatibility?
Opus was not designed to obsolete Vorbis, only Speex. However, as a happy accident it outperforms Vorbis for bit rates of 96 kbps and below. For higher bit rates, it is statistically tied with Vorbis, primarily because the difficulty of hearing any difference at all between Opus/Vorbis and the original.
So, for any use of low-bitrate Vorbis (or mp3), switch to Opus. For high bit rates, use Opus or whatever you did before, whichever is easiest for you. As I expect Opus compatibility to overtake Vorbis compatibility fairly soon, I would make any new encodes as Opus, at 128kbps if mono and 192 kbps if stereo. Anything above that is not really useful except for perfect fidelity, in which case you should just use Flac instead.
> More in general, is Opus indeed the best audio codec ever for any sort of audio at more than 12 kbps, or are there tests that show that something else beats it in certain situations?
Last I checked, Opus is either better or statistically tied with all competitors at all supported bit rates above 12 kbps, and statistically tied with the original at the maximum bit rate of 256 kbps per channel, but there might be some study I have missed.
> What about efficient compression of surround audio, both for command formats, and for generalized surround with an arbitrary number of channels in arbitrary 3D positions?
Opus only support stereo coupling, but you can add several stereo-coupled channels in one stream (eg 7.1 surround would consist of three stereo couplings and two discrete channels). Not optimal efficiency, but better than Vorbis (which doesn't support coupling at all when there is more than two channels), and generally speaking good enough.