And actually since the reason for the derating is heat and the heat is based on I^2R, the maximum heat is when all three hots are fully loaded or when 2 of the 3 hots are fully loaded. In both of those cases, you will have 1200(R) watts of heat. If you reduce the current from 20 to 15 amps on one of the condutors the total heat would be 1050(R) watts of heat.
(b) is saying that if you do not have all 3 phases in the pipe the neutral is always a current carrying conductor for the reason Roger stated, taken to the extreme (a missing phase has zero current nulling the other two).
