About coldboot haxchi:
- The DS game it uses must be on NAND, so, if the DS game is on any USB, move it back to Wii U HDD before you reboot or turn it off, or it'll brick for sure;
- The hacked DS game with Coldboot Haxchi in it CAN'T be deleted/removed from Wii U or it will brick for sure;
Tip: put it on a folder that you'll be sure to not touch or use ( i put it and other useless Wii U system apps in a folder called 'Useless', but you can name that folder as you like );
- You can move the hacked DS game on Wii U main screen as you wish, just don't do the same on Data Management section of Settings app or, as you can guess, it'll brick;
- You can play online on any game you have installed that has any online feature;
- The must-have channels are: Homebrew Launcher and WUP Installer GX2, any other homebrew app you wanna install as channel is by your own wish;
- To download games, you should use Wii U USB Helper (
https://gbatemp.net/threads/release-wii-u-usb-helper-a-small-eshop-for-windows.446565/ ), but you must provide the url for it can install games. Since that program can now install 3DS games too, you must provide 'that iso site' urls for both Wii U and 3DS ( google can be your friend on that ;] ) on respective orange bars that you'll see when first opening the program after installation.
So, as you can see, i am user of all showed programs on this comment, so, as you can guess, you're safe if you do what i wrote here and have common sense about what you can and can't do on Wii U.
Have fun! =D