Thanks for your response!
I think I managed to get what I wanted (and no compilation errors) using just your "\new Voice { \voiceOne \melody }" in the \score block. The actual piece is a little different than this simplified example, and it seems like I don't need the multiple "\new Voice" statements in the "harm" variable, since I only need an extra voice for measure a single measure (9 of a 16-measure section), and using multiple new Voice statements would have meant using many measures' worth of spacers, which seemed excessive to me. But simply adding "\new Voice { \voiceOne \melody }" did the trick, and I may use your two-voiced example for other re-harmonizations, so thanks again!
Ben