For Bootmii boot2 to be effective, you need to make a backup of your NAND. If you don't have one, and your wii bricks, then you may be able to access the HBC, but you still won't be able to get to the system menu.
If you don't have enough memory on your SD card, then try moving all the files to the compiter so that all that is left os the Bootmii folder. If you still don't have enough room, then buy a bigger SD card.