the wii u has to format the hhd into order for it to store things onto it, meaning you had to have had a second hhd for wii and gamecube. With the injector, it allows you to inject wii and gamecube games into icons onto the system and is stored on the wii u hhd.
You need CFW in order to inject Wii and Wii U games.
Edit: There seems to be a way for Wii USB Helper to still work. I highly recommend this because of its gui. https://www.reddit.com/r/CemuPiracy/comments/8gj8z8/workaround_download_and_run_wii_u_usb_helper/
Wrong, number 1, it is possible to have Wii, gc, and Wii u with this tool https://gbatemp.net/threads/release-wiivc-injector-script-gc-wii-homebrew-support.483577/
Yes, you are good. However, if your WiiU is hacked, you can use WupHax so you don't need a game dics. https://gbatemp.net/threads/wuphax-v1-1-hacking-vwii-from-wiiu-mode.446786/