Those themes have made it one of those unfilmable movies for so long so I know it's hard but you have roughly 4-5 hrs total between the two films, I think there was some time to sneak some of that in.
Count Fenring and Thufir could be left out, but not the Spacing Guild. They're a significant player in the whole series of books, they were part of the entire take out the Atredies plan. As you said, explaining the importance of spice to the whole galaxy, I think it would give more to the viewer than just it being the planet the Bene Gesserit planted its Messiah seed in.
If they do a part three it could be and I would like to see Paul's conclusion and the introduction of the rest of the players in the Dune Universe.
sometimes u gotta leave ur inner nigger in the bank vault. - desus