Regarding the hight-nrow-lowbrow, middle-brow business, something J.B. Priestley wrote in Literature and Western Man provides a little perspective:
That excellent American critic, Van Wyck Brooks, writing as a young man just after the European War (as it then was) had broken out, introduced into an essay, to deplore them, those brand-new slang terms 'Highbrow' and 'Lowbrow'. (He was wise enough, even then, to realize that the common use of those terms, dividing people into two opposed classes and somehow disliking both of them, would do far more harm than good. It has in fact done literature a disservice.)
And so it has. And so did Macdonald. "By Cozzens Possessed" is very funny in sits (I remember reading it when it came out). But it isn't just an attack on Cozzens's novel. It's also an attack on other reviewers. One of its points would seem to be that only the preternaturally perceptive Dwight Macdonald has got it right.