It sounds like your plan is really solid. Your color progressions should work well over your army. I dig it. I'll try to speak to your points in order.
1. Dark and gothic. This one is actually not too bad to try work with. I tend to think of heavy shadows when I think gothic. Your black basecoat will help, as will your washing with darker paints. Something to consider for your color composition is that the white will really contrast a darker paint set. This is a good thing, but keep your white focused on the model - too much and you'll lose some of that brooding quality.
Another thing to consider might be to take some dark green, water it down about 4:1 and wash your greens with that for extra shading. If it is too dark, you can always bring back your top highlight for effect. That should help soften some of your transitions and keep the brightness of snotling green under control.
Keeping your metals dark will also go a long ways. If you're drybrushing, try to use tin bitz first, then boltgun, wash with watered down black, and highlight the bright spots with pure boltgun or chainmail. That'll help make your colors and flesh tones pop, too.
2. Mixing paints. I mix all the time. Try to get your hands on a little dropper bottle and buy some distilled water from your grocery store. For me, being able to consistently add water in measured amounts is great. Helps with the ratios. If you're using a dropper bottle for your paints, even better. If not, try to be consistent with your brush-loads (or whatever transfer method you're using).
I like to use a toothpick for the actual mixing and would warn about using your brushes to mix. It can work well, but if you're not careful in your brush-care, the mixing process with deposit a lot of paint in the top of the brush (the forte, if memory serves) and can wear-down your brush's ability to hold a point.
Keeping paint mixes similar is good, but perfection isn't required. I know some guys that'll make a new pot of a mix if they're using it on an entire army. I know others who swear that the variation in their paint mixing adds character to the army. I'm in the latter camp. I do, however, keep a piece of paper at my painting station to record my paint mixes if I think I'll need 'em again.
3. Watering Paint/Inking. This is a great technique and works delightfully well for smoothing out colors. I would start with at least a 5:1 water to paint ratio. 10:1 is a bit better. It takes some practice, but the softening of highlight transitions and the deepening of shadows goes a long ways.
GW's new ink set is actually really good. I dilute those ~1:3 (ink:water) with great effect. They've also lasted through almost 2 armies, so you don't burn through them particularly fast, which is nice.
Hope that helps!