The brisket is going to measure differently at different places. This can be due to thickness of that area, and/or amount of fat in a given area. I generally will place my probe in the thickest part of the flat; where the point starts. If I have room on my rack, I will place the probe through the side of the meat; if not I insert it from the top. I start checking around 195°F, by using the temperature probe. If it slide in and out of the brisket like a hot knife going through butter, then it is done.
An internal temperature of 200°F sounds about right, but by your description, you overcooked the brisket. When it falls apart while it's being sliced or when picked up that is definitely an indicator that it was overcooked, but it usually doesn't remain moist. Was your brisket graded, if so what grade. Walmart only sell select grade, and I haven't had much luck with select grades.
A quarter inch slice should stay together when picked up at one end, and should be easily pulled apart. Another indicator of a well cooked brisket is that a slice should easily draped over an extended finger. A reason for the tough bark is that it was overcooked, and wrapping would have helped; but can you tell us if you used sugar in the rub, and how much. Too much sugar can also cause this. Just a note for future cooks; you can wrap anytime after that bark has set. If you wrap late and the bark feels really hard, sprit it before wrapping. This may not always solve the tough bark issue, but it will improve it.
Some things you can do when the brisket is over cooked and you want slices is to slice it much thicker than usual. That may help the slices stay together. Or try cutting with the grain, you still may have to cut thick slices. Also chopped brisket makes a good meal. You can chop it up, and add your favorite sauce. Serve it on a plate, or as a sandwich, or over rice or mashed potatoes; etc..
