I only quote from my background in chemistry.. just sharing the information..
The reason why CO2 is not effective against chemical fires (e.g. Mg) is bcoz the heat produced by the oxidation of these substances is sufficient to break down CO2. And yes, Mg can burn in CO2.. IIRC..
Of coz.. practical situations maybe different from theory.. so.. thanks for sharing of the practical info
And I agree with you that the effective handling of the extinguishers is actually not as easy as it appears..