So, the way I see it the relationship between Arceus, the plates and Pokémon, in general, can be very vague.
First of all, I agree that Arceus as the original Heart that is all-encompassing should have all abilities as that version of Arceus is collectively all of existence combined and every single being is just a manifestation of a possibility that it already encompassed. So when talking about myths and similar things, I think that Arceus could include every possibility in the Pokémon world because Arceus is everything.
In regards to the avatars and plates in general, it's strange because the lore is strange. A lot of times when they are discussed it's talked about as if there was a single set of Plates. Of course, in the 12th Arceus recovering the plates was a big thing, the same with the HG/SS manga and in a way even Legends Arceus... and yet the plates still exist somewhere else and multiple of them are confirmed to exist at the same time.
For the anime, while the Draco and Splash Plates were being used in the Jewel of Life, it was also being used by Team Galatic in order to get the Spear Key. While the Jewel of Life was still a thing and Arceus was mad at humans, Arceus still should have helped the people at the Arche Valley not only sharing part of its power but directly leading the power of the Plates, including the Splash Plate, to seal Hoopa. And this isn't even counting that now Arceus showed the Fairy Plate and for some reason, Team Galatic was in control over the Flame Plate in the recent Arceus' special.
And this isn't even considering the games that showed multiple plates existing at the same time and even being acknowledged, with the ones in the Abyssal Ruins
being directly mentioned not only in gameplay but also in the Pokémon Mystery Files on the "I Love Pokémon Club".
The truth is that the physical manifestations of Arceus and the Plates are full of vagueness and sometimes clear plotholes that not only contradict the settings but even the story itself. Because for some reason Arceus was mad at the Jewel of Life and couldn't use their powers... yet Arceus still was using it to seal Hoopa and Team Galatic used the same plates in the Jewel of Life. Even not considering the anime, the games themselves also make a lot of use of "there are multiple plates", even with that being illogical with all the lore and whole propose for the Legend Arceus post-game.
In the end, I feel that anything in regards to the physical manifestation of Arceus is only there as a plot excuse from the eyes of Arceus itself, as in the Arceus' special they showed how easily Arceus could get the Flame Plate, and yet Arceus allowed the humans to work on that because Arceus "doesn't act and only wants the better for humanity and Pokémon", the same feeling given in the manga.
So, I can definitely see Arceus as a whole being able to use every power and the Plates having something to do with the creation of the universe and the powers every Pokémon has, although the physical manifestation and the plates sometimes have contradictory depictions even inside the same media, so if necessary I wouldn't be totally against a note acknowledging the contradictions even if we address that the contradictions and limitations seems to now be explained as "Arceus just wanted to test mankind and the Pokémon".
