One possibility to consider - what if Hana has decided to become a missing-nin herself in order to go find Hazou? With the Mizukage having been kidnapped (and possibly killed), Zabuza, Mei, and several other jonin dead Mist is going to need every resource it still has available for things other than watching suspected traitors. (well, unless they double down on paranoia) But I could see the situation giving Hana an opportunity to flee on her own, as the village wouldn't necessarily have what they needed available to track her down quickly. Hell, I could see numerous Mist-nin abandoning the village - with Yagura missing and Zabuza dead, anyone who stayed out of fear alone might have decided that it was a good time to get gone (especially with the increased likelihood of Mist's total destruction due to lack of S-class fighting power) - so she could just be one of many taking advantage of the chaos.
...or she could have been declared Mizukage, because why the hell not? Mebbe nobody else wanted the job in the current political climate, so they picked a jonin that is a political nobody.