i think so, as long as it's a channel you downloaded. if it's a channel that comes with the wii, you won't be able to get it back. then again, i'm not ENTIRELY sure you can, but it's worth the memory in my opinion (heck, i just did it yesterday).
Yes, as long as Nintendo keeps whatever game you delete on their servers. There has been no word of them ever shutting them down, but in theory, that's possible. If you're paranoid about it, you might want to copy everything to an SD-card before you delete it from the Wii's hard drive.
Yeah, you can, you just look under something like "Purchased Items" on the main Wii Shop menu and redownload them from there. I've done it a few times myself. You can also get updates for already installed software from there too (never known exactly what they update most of the time though).
And I can't see Nintendo's servers going down for a long while. But if they do, get an SD card ready or somethin'.