It also results in the instashield graphics replacing the main shield rather than overlaying it because of the shared VRAM. As it happens, that works pretty well with the classic shield. I don't know that it would with the elementals.
If you want to stop the shields from running their animations, within the double-jump code we were tweaking before, look for any lines that set $FFFFCD08. From the shield's point of view this is $20(a0), I.e. it sets the animation. That should keep them from kicking up rough.
As for the CNZ/MHZ error, I managed to recreate this by changing the line at loc_68D8 from
So you probably want to change that back to #1 if it's set differently.