I appreciate your point, but I think it's important to keep the buttons conceptually the same across all characters, and it's precisely because they are characters, not entirely different gameplay types like the characters in something like Adventure. They're slightly different approaches to the same levels in the same game. I don't think the goal of only using two buttons gives enough benefit to be worth the confusion of those buttons switching jobs around for one or two specific characters when gamepad users have four face buttons and kb/m users have, well, the entire keyboard and who knows how many mouse buttons available.
E: Also, I think there's an important difference between the hover, an action that the two most secret unlockable characters share but with different stats, and shield actions, something that's identical across literally every character in the game. You wouldn't put jumping on the spin button for one specific character. Having an extra button to use her hammer would be just as unique as, and much less confusing than, moving an ability every character has away from its normal button and onto a button combo that normally does something else.
E2: As point in favour of putting shields on a separate button, holding spin in air already does something for characters that spin, it makes them roll when they hit the ground. If you have a shield, you can't do that mid-jump without using your shield ability unless you hold spin before you jump and don't let go until you land, and a lot of shield abilities mess with your jump in a way you might not have wanted.