Yes, I've tested the (now region-free) EU motherboard with an original Bubble Symphony cart which works fine. The tearing issue seems to arise in scenes like the beginning of Gekirindan and Darius Gaiden Extra where extra textures are being used for special effects, so I'm not sure if these games are doing something that Bubble Symphony doesn't....
I was able to address the audio corruption (for now) by placing the ribbon connecting the multi board to the PCB outside the chassis instead of tucking it between the PCBs. Hopefully that was the cause...
I'll let you know if the updated patches from Darksoft address the tearing as well as the Zone D background issue. Thanks!