The interesting thing about imagining the world of the New Roman Empire following a negotiated peace is, that at this point it would come in 1945 with just about all of Mussolini's ambitions realized. Oh, for sure Britania is not under control, but without the support of Russia (or the US), it becomes a rump capable of only annoying the Med around the edges.
During WW2 there was a very strong (initially) movement within the US to keep it neutral toward Germany. This worked in part because of the influence and pervasiveness of German-descendants in the states. However, Italian influence at that time was even stronger in some ways, linked as it was to the rise of prohibition mafia (only partly Italian, I know), and therefore black money. Also, the Italian immigration wave was much more recent, with increased homeland ties accordingly.
It seems likely to me that throughout the Great European War the Italian mob and political organizations have been working overtime to shade Mussolini's actions in the best possible light. For example--the interventions in chaotic Spain, to restore order for the common people; the aggressive defense of Eastern Italy against the Yugo-Turkish alliance; the other Balkan adventures in response to mobilizations by Greece, Romania and Bulgaria; and of course British aggression against Italy's friend, Germany.
During negotiations, I'm sure that Mussolini could win further favor by arranging a territorial swap with Germany: France (Gaul) for formerly Soviet territory in the east, save perhaps for the provinces of Galicia and Scythia on the Black Sea. The French would welcome non-German occupation, I'm sure. Also, in 1948 Mussolini could announce the formation of the autonomous region of Israel, winning more points with Truman and wide swaths of the US populace. Historically, Mussolini (or at least the Italian people--including the bureaucracy) were actively blocking efforts by the SS to round up Italian Jews, an effort that only got going after control was seized following Italian capitulation.
And of course, who knows how Roman-German relations will evolve, as an increasingly insane, syphilitic Hitler fumes over Roman triumphs and German inadequacies.
There's alot to write about how Mussolini continues the war, since politics is war by other means.
--Khanwulf