I'm a beginner myself, but to answer your second question, steeping the black roasted grains is not a silly idea. Surely it will darken your beer, but are you after a darker color, or the flavor you associate with the darker color? In addition to darkening the color of your beer, steeping the black roasted grains will impart the flavor of the grains as well (and is actually normally the main purpose), and the amount of flavor will of course depend on the amount of grain you steep.
Regarding ABV, I would say that as a general rule that steeping grains is more about imparting flavor than providing fermentables to raise the alcohol content (although it does that to some extent). If you use some beer recipe software, you can let the software do the computations for you. I've tried the BeerSmith software, at beersmith.com, and with it you can play with the ingredients (and amounts of ingredients) and see the different effects of ingredients on ABV, color, and bitterness, for example.