If I bought the Super Luigi U DLC on the Super Mario Bros. U disk, if I sell it, will the customer have the DLC

It is like before. I called Nintendo customer service a few days ago when my hard drive was having problems, and they said that is still true. The account is tied to the console. If it breaks, the best thing to do is send it to Nintendo and have them fix it, because

1. It wont cost as much as buying a new Wii U
2. They can save your data, or at least your account.

That was told to me by customer service at Nintendo, and I've had to have a console repaired by Nintendo before, and it's always gone smoothly. All my data was intact and just like it should be.

While it may take a little more time to send it in, it's cheaper than buying a new console, and you keep all your account data. In my opinion, that's the obvious choice for when a console breaks.

No, you won't. The disc slot acts as a "variable" piece of software. If you check your save data in the Settings, everything to do with a specific piece of software (such as the Wind Waker HD, which I have both digital and physical copy), will be placed in the Wind Waker HD "folder," including the save-data for each user Mii and the software itself. If you want to have 2+ separate files, just create another user Mii on your Wii U. However some games, such as Game & Wario, are "universal" and all the save-data is shared throughout the Wii U. In order to get an entirely different save file for that, you basically have to get another Wii U (but Game & Wario has 12 in-game save files shared by everyone, just as the Wind Waker has 3 in-game save files shared by only that user) .