To Survive the Dance, Marsh Must Follow Spitzer's Lead
Analysis of moves Marsh & McLennan, world's largest insurance broker, must make to survive New York State criminal probe into charges it cheated clients; imminent resignation of chairman and chief executive Jeffrey Greenberg is seen as only first step; Marsh's independent directors are reportedly already running company and deciding its legal strategies; are working on package of reforms to show Atty Gen Eliot Spitzer that they are serious about changing corporate culture; have little choice but to follow Spitzer's lead; photos (M)