I don't see how there aren't enough buttons. And I'm pretty sure Nintendo's going to allow you to customize what buttons you want or don't want to use. But just looking at it, 1,2 are the standard attack buttons, D-pad acts as the controls, depending on how hard you press determines if the character jumps or not, trigger can be used to shield/airdodge, and grab is the A button.
And the rumble thing is just a lame statement Sakurai released to cover his ***. If the gaming world found out that Nintendo was going to create a new game with controls that are more suited for the GCN rather than their purportedly revolutionary WIImote which is supposed to be so universally adaptive for all types of people, then Nintendo will receive a lot of harsh criticisms. If they were going to create a system whose reliant on standard controllers, then why didn't they just create an updated Gamecube rather than investing so much money on a gimmicky device? Easiest way for Nintendo to deflect such criticisms is to make sure that every controller works.
And how well each of the different controllers work isn't something they'll directly address. Instead they'll say something dumb like, "try all of them and the best one that suits you is the one for you" when the game is actually catered to be played with the GCN controller. So in the end, the change in gameplay will be minimal.
Besides, same physics engine as melee means that things like wavedashing has to be kept in, since wavedashing is Nintendo's way of solving players accidentally ghosting through the stage. Instead, during the airdodge, the character's momentum doesn't push it beyond the stage but allows it to slide across surfaces.