I don't know anything about Pokémon Presents. I only got info about Masters. Not about the trailer, but the story chapter itself

The new Arceus Arc will focus on Unova. The story starts with Iris defeating N using an Arc Suit in the Arceus Cup, and Alder wonders if it's time to fully retire as a Trainer and leave things to the younger ones. However, Marshal steps in and says that Alder can still aim to be Unova's strongest Trainer. Volo then approaches them, and suggests that Alder might also try to awaken his Arc Suit, pointing out that Arc Suits are so powerful that Leon defeated a living legend like Red. However, Rei eavesdrops on the conversation

Alder then begins training with Marshal, and awakens his Arc Suit in a battle against Iris in the Arceus Cup, defeating her

At the end of the chapter, Rei approaches Volo and asks him if it was all part of his plan. Volo reveals that he learned that Alder was defeated by N while retaining the Champion title, only to "lose" it to Iris. Actually, Alder had already retired as the Champion, but after defeating the Elite Four, Iris still wanted to battle him as if he were still the Champion. Volo then wondered who deserved the Champion title between N and Iris in front of her, in order to push her to battle N in the Arceus Cup and make her awaken her Arc Suit, and then push Alder to do the same against Iris

Rei wonders why Volo wants as many Sync Pairs as possible to receive Arceus' blessing, even though during their battle at the Temple of Sinnoh in Hisui he couldn't stand the thought of anyone other than himself receiving such a blessing. He doesn't know whether to stop Volo right away, or wait for Arceus to make a move

Arc Suit Alder & Volcarona and Arc Suit Iris & Hydreigon are probably the Sync Pairs for 5.5 Anniversary. I don't know about a third one, but maybe there could be a Story Event unrelated to the Arceus Arc, with another Anniversary Sync Pair? I only heard about the Arceus Arc part, so I don't know