1) That depends on how it gets bricked and what, if any, brick protection you have. If you have Bootmii installed as boot2, you can recover from nearly any brick.
2) Bootmii allows you to make a backup of your NAND. Provided Bootmii is installed as boot2, it will still run and allow you to restore the NAND backup if your wii ever bricks.
3) Don't mess with any NAND modifying programs unless you know what you are doing. Of course, if you have Bootmii installed as boot2 and a NAND backup, there is nothing to worry about.