We now know that the shoulder will be visited in a special epilogue, where Shulk and Melia travel there. Also, he apparently crafted a special Monado for the task (and that Monado bears some resemblance to Pyra's Aegis Sword, specifically the "vents" on the opposite side of the blade)...
The cutscene that showed Alvis in the trailer is from the part where he brings them to that one part of Makna Forest that leads to Valak Mountain, which is right after Shulk gets the Monado II and only about halfway through the game.
I don't think Alvis' new necklace is an explicit spoiler unless you know about the true nature of the Aegises. In other words, if you've yet to complete Xenoblade 2, you could just interpret it as a consisency thing (or if you've seen what Alvis looks like before, a reference that was added retroactively.)
Even if you've completed the original XC1 and not XC2, anyone who's seen XC2's marketing or has played a little bit of it know that the ones with zohar-shaped core crystals are powerful beings, and Alvis kinda gives off that vibe from the beginning, so the worst that could happen is that they'd add two and two together and figure out that not only is Alvis an aegis, but that the two games' universes are connected somehow.
I don't think there's anything inherently wrong with Xenoblade 2's artstyle, just that most of the time it looks so jarringly different from that of other gaems of the series, even Xenoblade DE's. (Even though that is constantly criticized for its "anime" artstyle...)