wow you're sure determained to get this right, im liking your views on this you obviously have the vision for what you want it to be but are just trying to find out the right way to do it. You really just need more concentrate on your forms, its like you're placing colours without full thought to how a face would react to light..
I like the version that was overdithered, apart from it being overdithered its got a lot of promise, the latest one is hitting some complicated lighting which really messes with the volumes. I think if you just blobbed everything out it would be simplier to construct and you'd be able to adjust the palette accordingly and make full sense of why you're doing it. I'll make an example just for kicks and giggles;
Im most likely going for a different approach than you but I'd stick to your style of mind, work out a set of colours that steadily drop to black, with no jumps or dramatic changes in hue. And place in your shades using the palette to build the characters form, very roughly done mind you. no fancy dithering or AA, just keep in mind your light source and trying to think 3 dimensionally, I'd also use your shades to replicate the beard from your sketch by bleeding the skin into the beard making way for hair detailing later, dont be afraid to break out of just colouring in the lines.
If you want hue shifts and dramatic lighting work on the form first and then work in alternative colours and trial new things.
Any dithering or AA should be done when you're happy with your forms, AAing mostly for finalising then whatever changes you need to make.