You have the right idea and are asking all the right questions. I think DCM is the right way to go but you will need to use both CTs, if you want to get an accurate number with mixed 120V/240V. The single CT approach only really works when the 240V “device” is balanced (no neutral wire).
I have a similar setup to what you are proposing - a floor heating subpanel with multiple heating loops on it. It does have a neutral wire on it with one small 120V loop and 4 large 240V loops, but I have knowingly used only a single CT, because the 120V loop is on so infrequently. I’ll have to look back to see if any of the individual heating loops have been detected - the DCM setup isn’t on my main Sense because the main ones uses the Flex Sensors for Sense Solar.