Max Brod seems to have been a super nice guy, but you have the sense he wasn't the brightest bulb in Prague.
While it seems to leave the playfulness intact, his editorship of the diaries butchers Kafka's radical structural freedom. I'm guessing there are two reasons:
First, he's working within the old scholarly ideology of the "canonical text," the smoothed-over, perfected, contradiction-free ideal which Macherey might consider a variant of the Normative Fallacy. He whittles away like a sculptor at the stone, chipping off a sketch here or a story there, leaving what he calls the "true diary" portion of things without fictional intrusions.
Second, he's struggling to corral Kafka's incipient "Postmodernism," as we'd call it now, back into a more respectably Modernist playpen. Brod can grasp his friend's dreamlike stream of consciousness. But he rebels at the intrusion of multiple texts within the text, multiple worlds within the narrative world. He wants the "real" world to be alone in this work, where Kafka wanted many worlds to exist side by side.
Fortunately Brod was incompetent enough to leave behind a sense of what K. was about. For instance the story fragments of July 1910 in which the narrator offers a critique of his education. At one point he tells us, "Perhaps my youth was too short for that, in which case, now in my forties, I still rejoice over its shortness with all my heart," to which Brod appends the pedantic and mystified footnote, "Kafka was twenty-eight years old at the time." Kafka's not speaking here. Brod misses that point, which is why these fragments are here for us to encounter.
Brod seems to have had a limited, 19th Century, bourgeois imagination. Kafka's anarchism was too explosive for his comfort. Kafka was like Sterne: he wanted to dynamite literary convention. Where Sterne waives his dynamite like a black flag, Kafka simply allows it to go off matter-of-fact, integrating it like background noise into his otherworldly narration. In Kafka the boundaries of convention aren't so much obliterated as playfully ignored.
Kafka was less the depressive and more the bad boy than is often noted. Brod's stewardship of his works has much to do with it.