Devas have much longer life spans than humans. So some devas who had seen Buddha must be still living in the heavens. Even the ones who had attained "maarga phala". Can they preach dhamma? Even brahmas who had attained some "maargaphala" are still there such as Sahampathi brahma. Can they interact with humans in some manner and preach dhamma?

They still can exist. But there is no mention that they can explain the dhamma to the humans and which is highly an impossibility because of the differences between humans and devas. They might be happy if a human spreads the true dhamma. Regarding the spiritually attained devas/brahmas, we may not know whether they already passed away from deva realms and had reborn in the Brahma worlds or already attained Nibbbana.