Nice plan - once everything is in place, would a small amount on boots/figures help tie them in?

Don't sweat the faces, think of them as pumpkins with some light shadow underneath, and highlights on top... just the way they are lit in your photos! 

I saw a guy who just brushed down (using a square edged brush) lightly (literally a glancing stroke) with the thinnest of paint for the highlights, and up for the shadows.

Like a lot of folks, I have a natural aversion to drawing on a photo of a face, especially one I care about. Sometimes I feel like the same thing applies to my figures, because I feel so invested and don't want to mess them up.