You're right that some of Welles' hour-long radio dramas are hard to sit thru, and I wonder how much Welles put of himself into them, especially the Campbell Playhouse shows. It seems that once the Mercury got a sponsor, they were obligated to make that sponsor happy, which likely meant less creative control for Welles, and less emotional involvement as well. After the Campbell Playhouse, Welles rarely worked in the hour-long format again, preferring 30-minute and 15-minute programs, many that I think are more interesting then most of the Campbell shows. Still, there are some gems among those hour programs, and I find them easier to get into when I put them in a walkman and go walking or jogging on a bike path or something. That's why I like to have them on cassette.
Yes, Vidal's series forms an unvarnished Cavalcade of America, focusing not so much on historical events themselves, but on the political scheming behind those events that, growing up in the American public school system, I don't remember ever being told about. In fact, I'd say I learned more from reading Vidal's books then from 12 years of history classes in public school. Like Citizen Kane, these books also shine a light on the growing power of mass media to mythologize and trivialize historical events, to make the people easy prey for demagogues and manipulators of public opinion. That's why Hearst is such a significant character in both EMPIRE and HOLLYWOOD. As Vidal puts it, Hollywood picked up where Hearst left off: whereas Hearst could only define what news was supposed to be for the people, Hollywood could define what PEOPLE were supposed to be for the people.