Yes, it is amazing that they came out as well as they did, undoubtedly due in large part to the superb sense of ensemble that the Mercury had, and from Welles' fearless juggling act as both star AND Master of Ceremonies.
I've heard that many 19th century novels were first published in serial form and that their authors were paid by the word. It's not surprising then that so many of these books are so massive. The authors probably didn't want them to end anymore then the readers did. Kind of like a TV series today, some of which go on a season or two longer then they should. I suppose the greatest novelists knew how to maintain the arc of the story no matter how long it went on.
Interesting that "Count" is the only Dumas that Welles and the Mercury ever did, although Welles did later star in Cagliostro aka Black Magic, based on Dumas' Memiors of a Physician. There was, furthermore, a 30 minute TV show that Welles did in the 50s based on the life of Dumas. According to Welles, the show was lost. Too bad. Bret Wood's bio-bibliography of Welles also lists Guy Endore's wonderful novel about Dumas (called KING OF PARIS), as one of the unrealized projects. The novel accuses Dumas of having signed his name to a lot of ghost written books, which is how he managed to "author" so many. Maybe the theme of ambiguous authorship is what attracted Welles. It's an issue he was bitten by himself.