About the fate of the colonies, more than Eritrea I see Somalia more favorable to remain an Italian possession, after all OTL Italy obtained trust territory administration from the UN until 1960. Libya is lost anyway but maybe Italy can obtain the recognition for its colonists to remain without harm (becoming a relevant minority), which could be useful later to have a claim to intervene if Gaddafi rise and he will start to expel Italian citizens.
Aside the PCI, the possible real enemy for monarchic Italy could be Tito... Surely he will attempt to raise the anti-Italianism in Yugoslavia to untolerable levels, in order to favour the national cohesion, starting with the issue of Trieste. Maybe TTL he didn't left the alliance with Stalin...
Umberto could have still various cards to his side to regain favour and influence. For first, De Gasperi and the DC are now forced to cooperate with the King and keep the actual status quo; second, surely Pius XII will support Umberto in full, and so the Americans, so the risks of a civil war will be as OTL frankly reduced, albeit I bet the (probable) attack on Togliatti in 1948 could create more destabilization with the party surely accusing the monarchists.
Plus, he could count about the popular support over Maria Josè, and after 1948 of Elena of Montenegro, which after the death of VEIII will probably return to Italy. The Italian Queens always had large popularity after all. However, a problem could present about the burying place of the former king. Probably Umberto will propose Superga as compromising place, but it could also be he will attempts to force the situation proposing the Pantheon as well; considering the monarchy survived, plus VEIII was still the one under his reign completed the Risorgimento (which it will be considered meritable enough respect to the decision to support Mussolini), he could be buried in Rome as well. Of course, the PCI will protest over that decision...