I got the impression from the progression of how Ighigo's zampakto's manifestation went from the old man to his inner hollow and the explanation that it was the state of Ichigo's emotions that dictated which one seemed to be in charge. I agree that the zampakto's wielder's reiatsu is what manifests itself into said zampakto. However regardless whether the reiatsu comes from a shinigami or a hollow, Vizard or Arrancar reiatsu is reiatsu.
The difference to my understanding was in the nature of said reiatsu. However as each zampakto has it's own personality separate from the one using it with it's own reiatsu isn't it a reasonable leap that for shinigami it's an incarnation of their inner hollow they can tame, exist and fight with. Something that they may have been doing without realizing it? Considering that Arrancar, in order for them to create their own zampakto they have to compress a large part of their own reiatsu.
We have seen hollows take the form of animals before, why couldn't Zabimaru be the formation of Renji's inner hollow? He said himself that Zabimaru wasn't that smart, the more base the hollow the more animal like it becomes. He's white, just like a hollows mask or Zangetsu (present, namely Ichigo' inner hollow) and we haven't seen the incarnation of any other zampakto but Renji and Ichigo's to compare them to. It's a fair leap.
While we are on the subject of Renji's bankai, I don't remember in any of his fights Zabimaru ever showing any other levels of bankai. Renji could remove the reiatsu from the pieces of Zabimaru to protect it from damage. For that matter Zabimaru was in shekai form right up till Aizen tore Zabimaru to pieces. That last desperate attack sending the pieces of Zabimaru at Aizen wasn't bankai. Now we have seen first releases take on more than one form, other than Byakuya no one has shown a second form of their bankai. Kaname Tōsen showed 2 forms of attack while in shekai, Tōshirō Hitsugaya has also showed more than one form of attack while in shekai. However multiple bankai, I can't remember anyone else doing it.
I do agree the expression that Byakuya showed at Ichigo's transformation was surprise, but can we really be sure he wasn't just surprised that Ichigo had achieved the transformation to Vizard? He showed the same look of surprise when Ichigo released his bankai. It seems many of the things Ichigo had achieved "surprised" Byakuya by this point in their fight. He did make a point of saying that he wouldn't "ask" about Ichigo's transformation.