Let me see if I can sort this out:
First, the Tripoli events were never part of the European Championship of the CSI/AIACR.
Second, that the 1939 GP di Tripoli and the other major events in Italy were to be an event run for the Formule Voiturette
was scarcely a surprise announcement by the RACI. The club later relented and were planning to run the GP d'Italia to the Formule Internationale
, when Hitler invaded Poland, that was that. Although Daimler-Benz did consider using a machine based on the Typ W165 as the basis for a return to Grand Prix racing, the formula change announced in 1951 placed that idea on the shelf and a new design was developed instead.
Third, the CSI regulations governing the Formule Internationale
and the Championnat du Monde des Conducteurs
(the World Championship for Drivers) were two, quite separate sets of regulations. Although Alfa Romeo did not formally withdraw from racing its Formule Internationale
machines until early 1952, it had already begun reducing the staff in its racing department. In 1952, the organizing clubs began to move to F2 as early as the turn of the year, although some did not formally opt for running their events to that formula until as late as May. The CSI awarded rounds of the Championnat du Monde des Conducteurs
to the national sporting authority which then either organized the event itself or delegated it to one of its member clubs. There was nothing that stipulated that the event had to be run to the Formule Internationale
, although that was the clearly the intent. This was why the Championnat du Monde des Conducteurs
could be run to F2 in 1952 and 1953; it is also why the US national sporting authority until 1955, the American Automobile Association, and then the ACCUS (Automobile Competition Committee of the US) in 1959 and 1960, could include the International 500 Mile Sweepstakes at Indianapolis as a round in the Championnat du Monde des Conducteurs
without it conforming to the Formule Internationale
, although it was the only run in the 1952 and 1953 championships that actually included machines that met the requirements of the formula.
Fourth, the Mille Miglia story regarding the MGs is, to be polite, folklore -- another way of saying it would be "myth."
Fifth, the change in the maximum displacement to 3-litres for the machines participating in the Championnat du Monde des Marques
beginning with the 1958 season was not a surprise announcement when the rules were formally presented in October 1957, the teams being forewarned in the late-1956/early-1957 timeframe that changes were going to be made in the marques' championship. The rationale behind the change was to rein in costs, reduce speeds, attempt to have the machines more closely conform to the notion of being actual prototypes for future production cars, and encourage more marques to enter the championship. The changes were also the first move towards centering the marques' championship around GT-type machinery versus the fendered-GP machinery then being fielded.