Regarding Jack's truck facing the wrong way when they parted at the end of the summer:
I don't think that was an edit error, just a loss in continuity because of a deleted scene. My understanding is that they did not leave for Brokeback the same day the met Aguirre...they spent the night in the truck, and then drove up to the trailhead and parked there. Then they came back down in the truck, and Jack must have parked it in rear first. You see that scene in the original movie trailer.... Ennis is sitting in the passenger seat. One has to wonder though why Jack would park his truck rear first regardless....seems to me that he was just dropping Ennis off, why did he even have to turn off the ignition? Oh well, we'd go nuts ironing out the details.
Regarding Alma Jr and Ennis confusing Troy and Kurt: This is also not an error to me, but actually yet another very poignant touch. Remember in the divorce hearing that Ennis only has to pay child support until the girls are 18...I would presume also when Alma Jr got 18 then mandatory courtordered visits would stop also. Therefore, Ennis probably hasn't seen her in a while.....note how he asks "what's the occasion?". So Ennis, desperately trying to stay close to his daughters as they grew up, gradually loses touch with them, and it comes to a point where he presumes that Alma Jr wouldn't visit him just for the heck of it, but there has to be a reason.