I have used fangs and bandit sashes to take an agnostic dark elf enchanter to the point where even the paladins are apathetic and none of the corrupt guards got upset with me. I suppose it might be that I use human illusions (which they seem to like) and dark elves are considered allies when I forget to cast it, but have you been following up the fangs quest with the moonstones quest in Fish's Ale? It doesn't help your corrupt guards faction, but it does add extra non-corrupt guard faction, meaning that you can get away with more of the corrupt quests.
McNeal Jocub (who wants the moonstones) gives you
a quest to go and collect a shipment of Blackburrow Stout; this improves the corrupt guards faction and the second half of the quest corrects the negative faction hits with non-corrupt guards. The other easy one is to go and
grab the tackle-box for Faren (on the dock where the boat does not arrive) and then
hand in the dress you receive to Ton Firepride for corrupt guards faction. The second part of this requires illusions, invis or good faction with the Bloodsabers, so you might just want to destroy the dress. (If you can hand in the dress, one of the rewards is a ring which you can hand in to the paladins for faction)