Umberto certainly does have the right to dismiss whoever Mussolini's successor is and he could potentially become the ITTL Juan Carlos. But the army ITTL will likely have genuine respect for Mussolini, since ITTL he wasn't the failure who became a German puppet, but the man who led them to victory after victory and made Italy a major power. If Umberto wants to change the system, the army might need to have a word with him.
That assumes the one party state has that level of support still decade after Mussolini's death among the military. His successor may prove less popular ultimately.
So far I like the sound of the South Korea option best. It seems to fi as it seems like a very reformed fascism will carry on post One Party State.