I know I'm too late to help, but...
I've done briskets about every way that has been mentioned. I have resorted
back to the way I started doing them in the beginning. Cook'em whole, no foil till done. Then separate flat from point and return point to pit for burnt ends.
I've never had a brisket plateau that long, a few hours yes, but not 6, but then again, each brisket is different. I would reposition the temp probe to see if maybe you were in a weird fat pocket or something?