Why is there no plural here for "chocolate"? If it were "big cakes," I think it would be עוגות גדולות. How is this different?
because this is a construct state and "big cakes" is not. a construct state= two nouns create a new word. chocolate + cakes= chocolate cakes. big cakes is adjective + noun= it is not a construct state.
הלא גם " הילד רק אוכל עוגות שוקולד" הוגן?
To me it sounds sort of... correct, but not very correct. It would be better in the sense of "the boy does nothing but eat chocolate cake". The word רק modifies the element directly after it.
Why is "אך ורק" not good?