Just goes to show whats been stated in the DT forum all along:

a) Niacin and cranberry juice does nothing to rid the body of THC.

b) Drinking a bunch of water days or weeks before the U/A is of no help. Drinking water is only helpful on the day of the U/A.

Chalk it up for experience. It you wish to keep using and are facing employment-related U/A's, subbing is the way to go.