Not a brisket expert, but I've done a few in my Amerique. I think your idea of 12 hours is way too optimistic. At 225, I use a rule of thumb of 2 hours per pound, but I've never done anything over 10 lbs. Also, I never wrap; bare the whole time (I like dark and almost crispy bark). At 250 as you propose, I don't know. There are still some real brisket experts who occasionally lurk here, so maybe one will chime in. My advice: cook till 200 - 205, then FTC for as long as you need to.
As for fat side up or down, I have read the arguments either way, but I do fat side up, since the fat will baste the rest, and the point will get done first anyway.