[pkscout]

Now, the problem.
Let's say the current profile is DIGITAL (HDMI), TV is off, amp is on.
Using Chorus web interface from a PC, I select an album and hit play. The playback sometimes does not start at all and sometimes starts somewhere in the middle of the first track of the album (sometimes also second track). Anyway the auto switch sets the audio aoutput to ANALOG (usb audio), but my impression is that it does so AFTER the playback has started, causing the issue. After that first glitch, if I do not hit the stop button (that triggers the add on to switch back to DIGITAL) the playback works fine.

Dunno if my impression is correct, hope anyway I explained the issue.

Your impression is correct.  That audio profile change is triggered when Kodi sends an OnPlaybackStarted event.  I don't know why Kodi isn't sending that event when you start playback in your use case, but that's the issue you need to figure out.  There is no work around on the audio profile side for that.