To answer some questions (hopefully all):
-- The plant tied to the rock is Anubias var. Nana
-- Salvinia doesn't cover the surface the same way Duckweed does... It's not as dense and does a LOT of pearling. It spreads much faster too -- I remove a golf ball sized portion of it every day from this tank.
-- There is plenty of open surface among the Salvinia for Groucho to take in air. Though I've noticed that this Betta doesn't really do much of that.
-- Yes, he can still build a bubble nest when he chooses to. Groucho likes to build his nests underneath the Salvinia leaves. He doesn't seem to care that the plants are in the way -- or maybe they're not in his way
-- No, a Betta cannot push healthy Salvinia plants upward. They grow in a chain which simply outweigh the fish. That's part of the reason I've chosen this plant species for floaters -- it takes away the fish's ability to jump out of the tank. Keep in mind this tank has no cover at the moment. Groucho can stick his "nose" up in between the plants to take in air if he wants to, though.
Hope that covered all the questions... If there are more, please feel free to ask