Dear Annie, just to throw a penny in the ring:
-- CITIZEN KANE, like all major movies, was a group project, but the standard Mercury Theater contract, which "Mank" signed, when Welles took him on to write radio scripts, gave authorship for anything he produced to the Mercury Theater in the person of Welles.
-- Although Mank remembered that Welles originally wanted to make a life of Alexander Dumas in a March of Time format, Welles freely credited Houseman with the "News on the March" sequence.
-- George Schaefer, CEO of RKO, is credited with the title, CITIZEN KANE.
-- If Mank was the sole genius behind CITIZEN KANE, who cut out the love affair between Jed and Emily, the assassination attempt by Kane's son against his grand-uncle, the President and the son's death in Rome in a Fascist riot. (The latter details, references to Lincoln's assassination, perhaps, would have given the title a cruder allegorical meaning.)
-- There are numerous memos on the progress of the script, including a couple in which Houseman refers to Welles' revisions.
-- If you piece, various memoirs and documents together (and there are people here at Wellesnet who have done much more of that than I), you will see that the story is a mix of Welles' concepts and Mank's experience.
-- It is rather commonly agreed that Welles wrote the scene overnight in which Kane is stripped of his power, if not his wealth, and that Welles created what many feel the single most brilliant sequence in CITIZEN KANE (some say, among the greatest in Movies): The swish-pan history of Kane's marriage to Emily. Welles said quite modestly that he stole the idea from a 1931 play by his friend, Thorton Wilder, with whom he said he cleared it.
If you have not read my review of CITIZEN KANE, you may wish to consult it. The piece contains a longish section on the writing of the script, gleaned from lots of reading, research and love for the subject over the years:
May that get us started.