Using the S/PDIF input would be the logical choice as it has no internal A/D and will mix just as any of the other inputs. The D/As are part of the output ccts so this can’t be avoided if you want to monitor using the URs outputs or phones. The Mute, Dim and Mono are likewise part of the outputs ccts and are not available on the phones or the S/PDIF output.
Download the UR28M Operation Manual and have a look at the schematics and block diagram for more info.
Hope that helps.