But Alph can't use the S.S. Dolphin.
Except he can, he didn't in Pikmin 3, but, he'd surely be able to pilot it if Hococate Freight were to rent it out to him.
And the Koopalings can't use Shadow Mario.
Isn't Shadow Mario activated by use of a special mask with Mario's nose and mustache drawn on it, rather than being an ability inherent to Bowser Jr.? Bowser Jr. might let them borrow the mask and paintbrush if they had a good reason for them.
Now, though, I do have to wonder if, canonically, Dark Pit actually would be able to handle the Three Sacred Treasures? I mean, even if the Treasures were designed such that only Pit could use them, he should be able to use them anyway, as a clone of Pit, unless they have some kind of built in test of one's intentions or something that he would fail.
But... the Three Sacred Treasures shouldn't exist in Smash 4 anyway since they were destroyed in Uprising and Smash 4 canonically(?) takes place after Uprising, so... it is really inconsistent, then.