I haven't seen any memos from "Bread and Circuses" indicating a massive fight. There's a letter for WGA arbitration in which the two indicate they want to share credit 50-50 for the episode which suggests the opposite.
Inside Star Trek went into rather intimate detail about Coon's departure, anyway (and it wasn't a fight). I'm not sure Cushman can present much more than that. Considering the two collaborated on Questor only a few years later, as well as other correspondence, I find the idea that they had a blow-up pretty suspect. But, we'll see...