Nothing's been proven. None of those characters are Steve.
If they did that to Steve, that would still be creative license in changing his appearance, and given that Sakurai prefers to stay faithful to the original source material, that seems unlikely.
It'd be the equivalent of giving Wii Fit Trainer a different and more expressive face. The difference between Steve and WFT is that WFT has some of the most unique animations and attacks of the entire roster, accompanied by voice clips. Steve has very few animations in Minecraft, his attack moves are limited, and he has only one basic voice clip.
Steve wasn't designed to be a fleshed-out character. He was designed to be basically the bare minimum of an appearance and animation for a game that's mostly played in first person anyways.
I mean, Piranha Plant has shown us that anything is possible. But I think between Banjo and Steve, the choice is obvious to all parties.