It's possible the damage caused to the slot was from a previous botched repair attempt.
When you soldered the slot onto the mobo I assume you ensured the existing pins did not touch each other or the casing of the existing slot? or did you just remove it?
Without the Homebrew Channel it's difficult to run code on a virgin 4.2 Wii and all installers for HBC do require a working SD slot, although after this it is acceptable to use an SD device. AFAIK this has not changed and you will still need the SD slot when running any exploit eg. Bannerbomb, Smash Stack, Indiana Pwns.
Have you tried a different SD card?