<<Is a false negative on the B test a possibility?>>
Sure it is, just as a false negative was also possible on the A test. However, I'm guessing a false negative is even RARER than a false positive. So, the B test comes back negative -- it's time to give him the benefit of the doubt.
Drug testing is never going to be perfect. If Lagat is not using, then he's an innocent victim of a not-perfect system. If Lagat was using, then he got lucky and if he's smart he'll stop using. If not, he'll get caught eventually.