Okay, so now I have Imported albums that are SACD ISOs with both stereo and multichannel versions of the tracks.
I tried playing one of those, and it only puts the stereo tracks into playing now. When it gets to the end of the last stereo track, it stops.
I tried putting the Multichannel Zone at a higher priority than the Stereo Zone in ZoneSwitch, but no change.
I tried playing something in Multichannel Zone right before playing the SACD ISO, but no change.
So, with ZoneSwitch, MC18 is now (apparently) just playing the tracks that are in the same zone as the first track in the playlist - meaning the playlist that is created by MC18 when it reads the SACD ISO.
For whatever reason, MC18 seems to always put the stereo tracks first, so the current behavior with ZoneSwitch is to only play SACD ISOs in stereo.
And in fact, ZoneSwitch says "The first file in a playback set determines the rule." So, SACD ISOs can never be played with ZoneSwitch if the user is a surround music fan who always wants to hear the multichannel version (and I know lots of them, and they would like MC18).
So, ZoneSwitch does not remove the need for an "SACD preference" setting that is similar to the "DVD Language preference" setting.