Then this Roon Ready device is behaving in a compliant manner for this aspect.
As explained by multiple posts above, Roon Ready = RAAT over network.
The limitation you found with USB does not affect Roon Ready (network).
If the device supports volume control with its native network playback, then Roon Ready requires it to support volume control over RAAT over network. This is known as feature parity requirement of Roon Ready.
If only if the device supports RAAT over network. USB is irrelevant.
No.
In a more complex configuration, you may have a network RAAT device that outputs USB (e.g. Pi or another computer doing a Roon Bridge), then USB input to the DAC.
In this case, the USB DAC is not functioning as a Roon Ready device.
Since you’ve got Roon Ready over network, why would you use USB when playing Roon?
Roon Ready only concerns RAAT over network.
USB input is irrelevant.
To my knowledge, Roon Tested (not Roon Ready) can cover inputs like USB, SPDIF, AirPlay, Chromecast, HDMI, or certain Roon blessed proprietary network protocols where RAAT cannot be done.
On the subject of UDP vs TCP:
Prior to June 2017 RAAT was based on UDP. Since Build 234 Roon redesigned RAAT to use TCP. See