A: The Wii U’s vWii (Virtual Wii) has an even stricter security model. Downloading a vWii NAND is guaranteed to brick the Wii U side, requiring a full NAND restore via hardmod. Never do this.
A: Only for developers testing region compatibility in controlled environments (e.g., dev Wiis with key writing disabled). For consumers, none.
A downloaded NAND backup will almost never fix a brick. Because you lack your unique keys (and boot1/boot2 hashes), the Wii’s security will reject the foreign NAND.