In case anyone's interested: I've solved, to my own satisfaction, the change of mode part of my question, in a rather unexpected way, by transitioning first to F sharp Aeolian. Of course, I'm still way off C minor, but modulating through minor-like modes and keys makes it easier (I hope). I think the main lesson here is that key transition doesn't necessarily involve advanced "chord magic" (although it can). Stylistically speaking, it's more about setting up proper expectations and then refuting those in a convincing way than anything else.
The repeat bars all involve two repetitions instead of the usual one, by the way. The key of G major had been very firmly established beforehand. I want to end with a modulation from C minor back to D minor (hence the key signature), followed of course by a protracted cadence in that key.
Follow-up question (to all you harpists out there): is the stuff I wrote for the harp at all playable, assuming a reasonably competent harpist? (The tempo is very slow, like 45 BPM.)