If they go this route for Necrozma and the UBs, I can see M21 be about Cosmog, Cosmoem and Solgaleo/Lunala with the Tapus.
The thing with Gen 7 is that it uses almost all of its legendaries as part of its plot, so you can't really tell which legendaries will be kept for the movies or the anime, since they are all connected in a way or another (The Tapus have the mission to protect Cosmog until it evolves into Solgaleo/Lunala. The Tapus have battled UBs in the past. Type: Null/Silvally have been created to combat the UBs. Plus, Cosmog and its evolutions along with Necrozma are speculated to be UBs as well. See? All are connected. The only ones left out of the story are Magearna and Marshadow, this gen's Mythicals which are already covered.
Doubtful. GF never reveals next-Gen Mythicals only two years in. It's more likely we 'll have another Lucario case before next-Gen Mythicals come into play.
It's a mess, really. They should have M20 being about Ho-oh only, and reserve Marshadow for M21. There's not even an event for Marshadow in Sun and Moon, like a NPC to get info about the Pokémon etc. Maybe the rumors are right and Gen 7 will be for 2 years only then Gen 8 on Switch? Then again, in Gen 3, they had run out of Mythicals for the movies in 2004 and the ones for 2005 and 2006 focused on Gen 4, mixed with the remaining Gen 3 Legends. Are we getting a repeat of Gen 3? That would be great.