It's possible that Astral separates from Yuma in order to save him from dying like what happened with Yugi and Atem, especially when it would be another reference to DM. I'm not sure how that would work exactly when they have the power of Zexal and both forms basically give them whatever card they want, so Number 96 would have to have something else to overpower something as ridiculously overpowered as that, but it's still possible. I'm still kind of leading towards something else like Don Thousand separating them for whatever plans he has right now.
Since that line was right before was about how the Numbers Club went off to learn about the Barians to cheer Yuma up, I assumed that the everyone's memories was referring them and not including Yuma. Plus, there is the next summary where Yuma is dueling for them, as you pointed out. While Yuma has dueled without Astral's help a few times, the summary did seem to make a big deal about Yuma dueling without Astral by his side and the episode will probably play it like a big deal too. Losing confidence is pretty much what happened the last time Astral supposedly died, so I don't know if they'd do that again. I think he would probably just be really depressed and possibly guilty if whatever happened to Astral is his fault. Number 96 really hasn't impressed me that much, but he generally works as a villain.
Also, the Shark vs. Rio episodes sound pretty awesome. It does make me think that Shark and Rio are the lost Barian Lords instead of their children, as much as I kind of like the later theory. The temple is where the missing Barian Lords are suppose to be sleeping and the visions that Shark gets during the duel could be explanations as to how they went from being Barian Lords to living on Earth. That really needs a good explanation.