Ah, playback,  Then I probably need to make this work.  As you may recall, the fundamental change in the code here is not just look at one big list of notes, but instead handle the upstem and downstem notes separately.  The code in layoutChord2 is only about sharing noteheads that have the same stem direction, and actually, by default, that won't happen unless user mirrors.  So my code here is for a notehead shared between upstem and downstem voices.

So I'll take a look at getting this working.