Brisket on the 22.5" WSM is fairly easy - main thing is leave it alone and don't rush it.
1) To foil or not to foil: depends on if you like bark or not. I used to foil, but now mostly do unfoiled. Since the butts will finish first I would cook them on the top rack and put the brisket on the bottom. This will give you the added benefit of basting the brisket for you as they cook - don't worry the brisket will still taste like beef.
2) Brisket will stall, I usually cook full packers (13-15 lbs) and 90% of the time they take 16-18 hrs. of cook time. But I have had one go as long as 20 hrs.
3) Don't spritz! Every time you open up your smoker you add approx. 10 minutes to your cook time - that adds up fast! Close the lid and leave it alone as much as possible. I usually only open my WSM to add wood chunks. Definately fill the water pan!
4) Rest for 1 hour minimum.... longer is better, I usually go for 2 hrs. Wrap in double layer of foil, then place in a dry towel lined cooler, fill rest of cooler with more towels - it will stay hot for 6+ hours that way without any issues. So better to finish early and rest a long time than finish late and not have enough rest time.
5) I usually cook brisket and pork butt at 250°, a rough rule of thumb is 1.5 hrs. per. lb., but every piece of meat is different and usually the 1.5 is a bit on the generous side. Your 20 hr. window should be good for cooking time and a couple of hours of resting time. If your brisket hits 190° internal and you still have over 4 hrs. to serving time I would pull it at 190° and keep it hot in the cooler for the rest of time. It will still do a little cooking, but by pulling it at 190° it won't get mushy on you from sitting in the cooler for so long. Same with the pork butts.
I usually start my briskets and butts at about midnight if we are doing them for dinner between 4 & 6 the next day. I have a welding blanket I wrap my WSM in so it doesn't have any issues with overnight temps or wind. Once I have the meat on I usually go to bed till about 6:00 AM, then check it when I get up. After that I just let it cruise till it's done and ready to rest.