Actually it's not really sprites anymore, only sprites we now see are in-battle.

I don't think they have a specific name beside 3D model. If we were allowed to customize out characters I guess we could call them Avatars, but we can't so we won't.

Ah, so the Notched-Ear Pichu does have its own battle sprite (as well as an overworld sprite as we saw before. Hm, I'm going to then guess this Pichu is either not able to be traded to Diamond/Pearl/Platinum or it'll revert to the normal Pichu sprite (but will revert back being traded back to HeartGold and SoulSilver... maybe (if the latter is the option they chose for the tradeability for the Notched-Ear Pichu)).

As for the Arceus event, sounds pretty cool and finally we'll get to see the Unown actually swarm like they do in the anime. However, according to Bulbapedia Arceus creates an Egg which will hatch into a Dialga/Palkia/Arceus (which kind of explains why they'll be at level one). Hm, I'm going to guess that Arceus combines his power with the Unown's reality bending ability to create the new Dialge/Palkia/Giratina egg so than afterwords it wouldn't have to fall asleep like it did when it created the original Dialga, Palkia, and the Lake Trio (still not sure if it created Giratina though). Looking at the last screenshot, the "portal" to the left is probably Dialga's being blue and it looks like Dialga's chest crest (which contains his Diamond jewel), the one to the right is probably Palkia's being pink and looks like its shoulder (where its pearl jewel is), and the upper one probably belongs to Giratine being the other two I already said belonged to Dialga and Palkia and also becasue I can't tell what it might resemble due to Gold standing on it.

Hey, wouldn't it be funny that after creating the egg, the original Dialga/Palkia/Giratina appears and starts a battle with you (however you'll be disabled from catching it). That'll be pretty funny and cool.