I'm back !
So, I was reading about the Peace Treaty: basically, we were royally screwed until Nenni, Sforza and De Gasperi basically achieved a diplomatic miracle and prevented South Tyrol annexation to Austria.
Had they failed, the De Gasperi government probably wouldn't have survived the political backlash. This would also cause massive outrage in the Christian Democratic religious base, heavily leaning towards pacifism, and in former fascists: some of them could even move towards a "national bolshevik" position.
So, we can have :
1) Unsuccesful "battle" for Bolzano
2) Soviet Union supports Italian claim to former colonies (it did, OTL)
3) Socialists and Communists don't try to unify, preventing the socialdemocratic split apart for the small reformist wing, which amounted for less than 10% of the PSIUP.
4) In 1948 elections, the Christian Democracy, identified with servilism to Americans despite De Gasperi's attempts, is defeated by PCI-PSIUP alliance, mostly due to a strong nationalistic outburst which benefits both the far left and the far right.
5) American and French troops occupy Sardinia, Sicily, Imperia and Val D'Aosta. Imperia is annexed to France, Sardinia-Sicily and Val D'Aosta are proclaimed independent republics.
I don't think that, in such a scenery, we would see Umberto II as the King of the revived Kingdom of the Two Sicilies, he was too much a patriot for such a thing. However, they could pull a Bourbon pretendant for Sicily, and keep Sardinia as a republic (with a fair chance of a communist revolutionary attempt later on), or make two banana republics of them. Val D'Aosta would be French-aligned, but basically a liberaldemocratic republic.
Things in Italy would then be ... interesting !

Even without Plan Marshall, I think we can get some help from UNRRA, like Yugoslavia: Soviet Union, to help Togliatti, could also lift its requests for war compensation.
Quite surely, the new left government would create a national myth centered on Mazzini and Garibaldi as the forefathers of Turati and Gramsci.
As for nationalization, I think that the government plan would look much like to the original center-left program in the 1960s : agrarian reform, workers' councils, nationalization of railroads, electricity, water, school and family law reforms. Decentralization is a tricky question, OTL the left was a strong proponent, to limit Christian Democracy's power, TTL we have to see.