I also have stopped buying Smart and Final briskets. I have had some great cooks with them, but, also some bad ones. I prefer Angus of Choice from Cash and Carry for bargain briskets. They charge a little more than Jetro, but, no need to be a business and never a big line.
Probe tender is a hard standard until you nail one. When you think it is like poking a hot skewer through butter (or perhaps probing pudding), you start to understand. The probe should almost insert itself. I actually use a shish kebab skewer. Many use an ice pick.
The rule I use is fat cap towards the source of heat. In a cooker such as yours, that means fat cap down. That is not why the flat was tough though. I would also say, that meat that comes off of the heat will always be a little softer, and then it will tighten up. A brisket that is properly cooks tightens up just a tiny bit. If it is over cooked, it won't tighten up at all, as the collagen and connective tissues are so denatured, they cannot tighten up. A piece of meat that tightens up a lot after being removed from the heat is undercooked.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."