I would believe you on most of them, but something feels like leaving Arceus absent for two consecutive "main" Regions would be something they try to work around (LA and BDSP were Sinnoh based and excluded several Galar aspects).The boring answer no one wants to acknowledge is those Pokemon won't be in the game
As for how this theoretically interacts with Tera Type, I think Arceus (or Silvally) would simply start out in the form of its appropriate Plate/Memory when entering the battle, but it isn't checked subsequently in the battle. Judgement takes after the Plate itself as mentioned, but Multitype seems to simply check "Arceus Type = Held Plate/Crystal" when the battle initiates and lock that in as its "Default".
Pokemon Transforming into Arceus or Silvally simply copy the Type and Form regardless of their own Held Item (after Gen 4 at least), so the check seems to only happen at one point before battling begins. By extension I would assume if I send out Silvally-Electric and Terastalize with a Fairy type, Reflect-Type or Conversion 2 simply treat it as a Fairy-Type for the rest of the battle interactions.
Jury's out on Transform, but that I would assume is mostly down to if Transform mimics Terastalizing at all. One could transform into Megas and certain Special forms like Ultra Necrozma, but would not mimic Dynamaxing if it copied a Max Pokemon, appearing as the base instead (and having to Dynamax manually if it wasn't Eternatus or the Dogs who can't), so there's precedent for Transform both copying AND ignoring the gimmick of a generation, not to mention it often has weird interaction with Signature Moves/Abilities.
That brings a thought to mind: what if Gen 9 removes Ditto to minimize Transform shenanigans (since it's the only one you can't keep if the move itself is removed) with Terastalizing? Imagine the fall out for Pokemon Breeders who need to get native Pokemon for certain early VGC events