I have noticed the difference in playback for different sources when using the WDM as well. I didn't analyse the cause, but it is one of the reasons I haven't switched to using the WDM for all external sounds on my HTPC. That and;
1. The lag on the WDM is very variable, and there is no separate voice sync setting for the WDM. (Yes, zones. Noted.)
2. For system sounds, the WDM activates, plays the sound, then deactivates, then won't reactivate for a couple of seconds, and if a sound is played at that time it is lost. So Ding [short delay] Ding comes out as Nothing, delay, first Ding, second Ding lost.
Mojave, it looks like you have a solution ... almost. If what you suggested was made to work, is it a better solution from a lag point of view than the WDM?