If the Wii Shop Channel has to redownload a ticket for whatever reason, it deletes the Riivolution channel. If you reinstall it, you should be fine. This usually happens, when you use AnyTitleDeleter or something similar to delete a title you originally got through the Wii Shop Channel, cause...
Oh yes, you can. There are modders that take out their disc drives and put an HDD inside the Wii. They wire it so it basically works as if it's plugged in from the outside, just less messy.