you NEED to restore the mii channel back !
Please follow the tutorials you read completely. Do not think half the job is enough or that you can mix guides together before completing them.
The things are there for a reason.
the "replaced Mii channel" is NOT the hack. it's not what allows you to launch homebrew on vWii.
it's only a replacement of the game disc exploit, which boots the "installation" of the homebrew channel, and you need to replace the mii channel back before using vWii normaly.
you don't need to run the "installation" process ever again.
Homebrew channel is already installed, you just run the channel from the system menu.
The Mii channel is used to edit your mii, and can be problematic if you don't restore the original executable.
Also, can I use my HDD from my old Wii, with all its games and saves, on the Wii U?
Yes.
Attention, if you plug your FAT32 HDD while you are still in WiiU Mode, it will ask you to format the drive.
never format it !
There is a Wii homebrew (and a Windows application) you can use to hide the drive from the WiiU and any other devices except the vWii homebrew : UStealth
Oh, and you'll also probably need additional USB power if your drive is not externally powered. The WiiU doesn't provide enough to HDD, so you need a Y-Cable.
You should follow vWii softmod guide from here, for example
this one, start reading after installing HBC.