The first track is the 5.1 Mix… is your copy somehow different?

Nope, that’s the one I have. Looks like I just went cross-eyed from all of the new information I was getting. Okay, so that solves that problem, I now know that I have the 5.1 on all three films. The other questions still apply (making sure I’m burning the correct track, and verifying that the 5.1 doesn’t get turned to stereo during the authoring and burning processes, and whether the MKV is sufficiently superior to warrant re-downloading and starting fresh).