1. No, its fine. During normal online play, HBC can't even be detected I believe. Provided you don't cheat online, you will not get banned from Nintendo WiFi Connection.
2. Yes, Shop Channel and purchases from it will work fine. At times when there are updates to the System Menu, a workaround may be necessary to maintain both access to Shop Channel and Homebrew Channel (HBC), but purchases will always work, without any workaround required.
3. If you have an old Wii, install BootMii as boot2 and take a NAND dump. The HackMii Installer (which you use to install HBC and BootMii) will tell you if your Wii is old enough or not. But if you're careful and don't do stupid things with your Wii, it shouldn't brick anyway. If you can't install BootMii as boot2, just be a little more careful. You could install Priiloader, but it will only aid you in minor bricks, which is still better than nothing, but no substitute for BootMii as boot2 if it is possible.