I believe that Welles' career disaster came not after Ambersons but before Ambersons; Welles was a great dramatic talent and director; John Housman was a great producer and Screenplay/script/playscript writer; both of them fulfilled these dual roles perfectly and symbiotically. Virtually everything they did together was at the least an artistic success, and often what they did together was a financial success;Houseman first saw Welles as Tybalt in Catherine Cornell's company's "Romeo and Juliet" in Dec. of 1934; about three weeks later they met and first worked together on "Panic". Over the next seven years they ran the Mercury theatre and produced plays, radio programs,record albums and films. Houseman claims (in his book "Run-Through") that he and Welles chose all the material together, and that he (Houseman) wrote ALL the scripts. (If any of you have read Welles' script for "Big Brass Ring", you know how important Housman was for him). And of course, Houseman was a very polite and patient man, who always did the business dealings, and we know how Welles was at those! Their list of accomplishments, for such a short time, is great: Panic, Heartbreak House, Horse Eats Hat, Caesar, Too Much Johnson, Dr. Faustus, the "Voodoo" Macbeth, Danton's Death, The Cradle Will Rock, Five Kings ( an early version of Chimes at Midnight), Native Son and others. To the end of his life, Welles always said that the Macbeth opening night was the most exciting night of his artistic life. And of course, the Mercury Theatre on the Air, with the famous "First Person Singular" series. And then Citizen Kane... a famous argument, and the end. The Ambersons was the first project that Houseman didn't write the script for, and it shows in the script and business confusion surrounding that project. Houseman wrote the script for Jane Eyre, but it was coincidental, as they were both just hired for the project (perhaps Housman was also the producer?) and the Mercury Theatre was in essence dead.
They didn't talk for years,but were finally reunited in the 1970's on the Merv Griffin show, where they waltzed with eachother, and fogave eachother.
Johm Houseman's memoirs Volume one "Run-Through 1902/1941" is an invaluable piece of Wellesiana, and (if you can find it) the Voyager cassette box that Richard Wilson put together shotly before his death entitled "Theatre of the Imagination" features a forty minute tape with Mercury members including Housman, Wilson, William Alland, Peggy Webber and others discussing Welles, and it's clear that, for all, the break-up of Welles and Housman was the major reason for Welles not fulfilling his potential as an artist, and for having (causing?) so many problems with producers for the rest of his life.
Of course, Housman went on to produce for Selznick and many others, got into acting and finally won an academy award for "The Paper Chase" (which must have driven Welles up the wall!).
This, for me, is the key.

