You're overthinking it. With 3.75 lb can of Cooper's Stout and 2 lbs of Dark DME from the homebrew store it will be plenty dark enough, and also have that stout-y taste. Plain ol stouts aren't very high in alcohol to begin with, 5.75 lbs of malt extract will work out just fine, IMO. Caravienne and Carafa aren't fermentable malts anyways, so you'll basically be wasting them on this particular batch.
There is a very fine line between "hobby" and "mental illness."