Nick, thanks a lot for your important answer. I will go ahead with my project in this direction.
Actually I did not know Pi Musicbox had a separate Spotify web client. I understand the reasons though. For me the MPD API (i.e. all the MPD clients) and the existing web clients such as Iris are still the major attraction to use Mopidy to control other device features.
I thought of providing a “more complete” version of the existing https://github.com/forscher21/mopidy-cd extension with the usability features added (medium detection/recovery, read-speed reduction, etc.), I am afraid that extension will not work without some of the patches/commits I will have to do regarding the existing audio class in my fork of mopidy.
Once more thanks a lot for your help.