using oneVoice vs force-hshift in polyphany and alternatives

[Stephen]

Sure, the notes are in the right place. The interaction between the various 
parts is baffling though. Using \oneVoice to set the notes over one another 
seems to make the stems go up; \stemDown erases the effect of \oneVoice and 
\oneVoice cancels \stemDown. Apparently \override NoteColumn #'force-hshift 
= #0.0 accomplishes the what I want without side effects.

What is \oneVoice meant to be used for? Can someone list separately all the 
things it does?

Finally, I don't understand the scoping rules at play here. How come I can 
comment out some of the stem overrides while leaving them in effect?

Stephen

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