You took your briskets out way too early. No brisket could be done at 145, and if your probe went in like butter, you must have been in the fat line between the flat and the brisket. Stacking the briskets is also not a good idea. Always make sure there is space all the way around each brisket so the heat and smoke can penetrate evenly. As far as the date on the brisket, that shouldn't have anything to do with yours being tough. If anything, it would only make it more tender, right up to the time it goes bad. And that you would smell right away when you unwrapped it. I wet age mine for 40 days in the cryovac at 36 degrees.
Every brisket is different, so there is no exact temperature that says it's done. Almost every pork butt will slice well at 185 and pull well at 195, but for brisket you have to rely more on feel. In any case, every brisket I've cooked (at least a hundred) was done at some temperature between 195 and 205 in the middle of the flat. I cook at 250. Guys using the high heat method might have to get the temp a little higher and guys cooking at lower temperature might get it done at a lower temperature, but NEVER below 185. We're not talking about a steak here, we're talking about brisket. It takes time and temperature to get that thing tender.
The main thing to remember about BBQ, but especially brisket, is "It's done when it's done."
Mister Bob, Pitmaster - Smokestack Lightning, Bad Ass Barbecue. KCBS CBJ #31759 - IMBAS Certified MOINK Baller