I only brought up the cast list, with regards to Fiennes, because when previous releases from Eon were brought out crediting him as 'M' (The theatrical Poster, Press-releases, Tweets from the Offical James Bond twitter account) the argument was made that we were waiting for the cast-list to see how he would officially be credited. As you say Prisonermonkeys, the intent is to present the information in a way that is representative of the subject manner: in this case Fiennes has consistantly been credited as 'M' in the lead up to Spectre in official Eon releases, it is in keeping with the precadent set by the earlier films in the Bond series and consistant with the film Spectre where he is referred to as 'M' throughout the film (and only refers to himself once as 'Mallory' at the climax.)
As SonOfThornhill suggests, the name Mallory should still be used in the character description (in much the same way Oberhauser is used in the description for Waltz's character), but a credit of Fiennes as 'M' would be more representativve of the film than the current credit.