Just for clarification when you say that “control of output level” does not work are you referring to the volume control or to the line output voltage? I think you mean volume but I just thought that I would check.
I note from the screenshot that volume is not set a maximum but lower. If the volume control does not work either using the rotary control on your Rossini or via Mosaic app how was this volume level accomplished? Is is just stuck there from a time when it did work? Incidentally where it is set corresponds on my set up to around -35dB which is really too low for full resolution of the music playing. A lower line output voltage would allow you to use the volume control ( when it works ) into your Pass power amp at a more suitable setting if where it is now represents an average listening level for you.
I also note the temperature of the Rossini which strikes me as marginally high which may be normal or may suggest a fault. None of my Vivaldi boxes exceed 40.
I am not sure where the failure to connect with Apple Play leads us as it seems that all other connectivity issues are working fine.
Failure of the volume control to work at all in any mode is not a fault that I have seen here and which can be solved via an online chat. I am assuming that you do not have the front panel menu display active when the volume control scrolls through the menu options instead of controlling volume? A similar scrolling not volume function occurs if you are playing from a USB stick when it navigates the tracks. If neither of those options is being used then as your Rossini is still under warranty I would contact your dealer. That is what they are there for.