Finally got the greyscale drawing finished.
The way I've been working for a while now is to do a completely rendered greyscale drawing before even thinking about color. I get all the problems of proportion, value and design figured out before worrying about color. The color layer comes next, right above the greyscale layer and set to "multiply". You can see that in-process above. Once the color layer is down, then I start on the highlight layer.
I've also been experimenting with the layer setting "soft light recently", yielding mixed results.