Version_401
I like this scenario. Kakashi may be stronger than Hayate but at the start of the story he is less alert and prepared, in addition to having only a small chakra pool and average swordsmanship, which makes him take poor decisions during the fight. Hayate is always very sharp, prepared and professional. Moreover, he is a specialist in the sword. He is in his element, which makes up for a lot in terms of power difference. And Ren's calculated interference gives Hayate more of an edge in this fight. So suprinsingly, they're doing pretty well. However, without his favorite sword, Zabuza will increase ninjutsu attacks. Normally they aren't on the same ground as in the canon, are they ? Which means, no immediate water point to boost his attacks (and no Water Dragon). I think in your fanfic there would be no reason for a second round. In the canon, Haku was intimidated by Kakashi's reputation, not knowing that Kakashi had little remaining chakra, so he shot his needle through Zabuza's neck to come back later and better prepared. But in your story, Hayate has no special reputation. Seeing that Zabuza cannot supplant the duo, Haku will decide to show up directly to aid his master, forcing Ren to actively engage in battle.