Flex and Derate cannot be used at the same time.
Down Three Greens.
Maybe not on the Bus but:-
JAR OPS AMJ 25-13 / AC 25-13
(4)(c) Reduced takeoff thrust, for an aeroplane, is a takeoff thrust less than the takeoff (or derated takeoff) thrust. The aeroplane takeoff performance and thrust setting are established by approved simple methods, such as adjustments, or by corrections to the takeoff thrust setting and performance.”
Clearly says that Flex and Derate is allowed. This combination is used on other aircraft types.
Otherwise your explanation is spot on.