Convert photo to 2 colours for printing (not duotone)

I have a lot of movie images that I want to prepare for printing with 2 inks (black + a spot red). In the finished product, I want everything that contains red to be in colour, while everything else is black. The colours can overlap. I want to find the best method of achieving the colour effect while being suitable for printing (image info spread across two channels). 

My current process is basically converting to Grayscale mode, then creating a spot channel and filling it with a Color Range selection that I got before the mode conversion. 

See attached for one of my attempts. I'm ok with the general result, but I think I'm missing one or two steps to get what I'm really after. 

The problem is basically exemplified by that rosy part of the dress. The full strength of the red is good, but the actual details are lost (underneath?). I'm not sure how the grey and red channels are interacting, since I can't change their order. I feel like I either have to adjust my selection method or change something about the black part of the image.