Idk abt wii u but I modded a new 3ds xl just this month. you basically put some files in a sd card and go to a website, and boom Homebrew!

You can, it pretty much revived most of the Wii U features like Wara Wara Plaza, Wii U chat, and Miiverse ofc

I just modded my cousin's old 2DS. It was easy since it was on old FW and I could use Soundhax, but it works great now

nintendo network shutdown only affected online games, not internet browser or internet features in homebrew apps

all current modding methods are not affected