1. Installing the Homebrew Channel and BootMii are completely safe, there are no reported "bricks" (i.e. broken consoles) from just installing HBC and/or BootMii. Other stuff (cIOS, patched System Menus, themes, "backup loaders") have been known to cause bricks, but the vast majority of those things are against the rules here. None of them are required for regular homebrew.
2. Legality of homebrew is probably a grey area. Technically, when you agreed to the Wii EULA (first time you turned on the Wii), you agreed not to run homebrew ("unauthorised software"). But as far as I know, the laws of most countries only really talk about piracy and bypassing anti-piracy measures (which would be the territory of backup loaders, not homebrew), not running "unauthorised software". And most people will tell you (correctly or incorrectly, I couldn't say) that the EULA is not legally binding. Note that laws vary from country to country.
Long story short, it is extremely unlikely you would ever be arrested for installing HBC onto your own Wii. No one else has been. But if breaking the law is a huge issue to you, you may not want to bother, especially in light of my answers to 3 and 4. (In all honesty, I would not let this bother you enough to prevent you installing HBC, but it is up to you). On a side note, please know that Nintendo will either refuse to fix a broken Wii if they find HBC is/has been installed, or else will charge extra.
3. Sadly, no. Due to the way the Wii works, there is not much capability to run code in the background while games etc. are running. The only way is a hardware solution, like this one: [
www.amazon.com] (thankfully they're not overly expensive). This sort of thing doesn't require homebrew at all.
4. You can uninstall HBC, yes. But Nintendo will always be able to find a trace of it's presence. If you're thinking about your warranty or whatever, it will be gone the moment you press install in the HackMii Installer.