Meh you guys are overthinking things. Typically Haki allows a haki user to bypass a DF users DF defenses like where Luffy loses his blunt damage rubber defense. So with haki any haki user should be able to cut Mr.1 or Jozu…........but the thing is Oda is most likely making some exceptions to the rule due to "PLOT" for hard substance DFs like Mr. 1 (steel) and Jozu (diamond) that even with haki you need to be able to strong enough to cut a certain substance. So if Zoro ever faces Jozu (or probably someone from Blackbeards crew like Shiliew that stole his DF since I don't really see a reason for Zoro to fight Jozu seriously) when he's able to cut him that'll mean he is able to cut diamond. Not becuase he used haki.
Mr 1's defeat could've been the result of a combination of Observation + Armament Haki, though. That long description that Zoro made about hearing the "sound of the rocks falling" wouldn't have been a coincidence.
Also, even under Mihawk's training, I doubt that diamond could be easy to cut through. I mean, the latter tried to test Whitebeard's strength, and met relatively easy confrontation from Jozu's defenses.