Upon further examination, the change I was planning to make to Audio::Mixer is not very useful without a related, but very complicated, change to AudioStream (and all of its child classes), that I am not prepared to make. And one of my mentors has informed me that the audio issues I’m encountering are acceptable. In the interest of not completely wasting time while I wait for a change on the status of the graphical solution, I will be working on a different engine in the meantime.
Specifically, it has been brought to my attention that broken sword 2.5 has a saving issue, having to do with serializing/deserializing pointers, that prevents save portability, and is the last blocking issue before the game can be called supported, so I will be looking to fix that while a decision is made on how to proceed with the HE scumm engine graphical issues.