I have never heard that niacin was problematic - except that pot heads take massive doses to "clean out the THC from their system." (Except that doesn't really work.) But, whatever.
It would be safer to get your state law changed, than to abuse your body.
That depends on many factors ... how often you use, when the last time you used was, how much niacin you use, etc ... some swear by it, others think it's useless ... nobody can tell you for certain whether or not you'll pass a test with it.
Can you tell Us more about what you need help with? What's a niacin mouth swap?