Your challenge is, without changing Italy's performance in World War I, to avoid both a Fascist and a Communist dictatorship in Italy in the interwar period.
Actually this is not really difficult. Simply have Vittorio Emanuele III crush the fascists during the march on Rome (1922). OTL this didn't happen because the king preferred to treat with Mussolini, fearing a possible cop d'etat of the duke of Aosta (allengdly a fascist symphatizer). This fears were completely unfounded.
So, had the king given the order, the troops stationed around Rome could have crushed the fascists without problems. The fascist were lightly armed and they would have had no chance against regular troops. Even if a part of the troops or of the officers turned out to be favourable to the fascist cause, they would have been too few to make any difference.
The outcome would have seen the high exponent of the fascist party tried and incarcerated, the four quadrumvirs hanged, Mussolini fleeing to Switerzland and any officer guilty of even thinking of betraying sent to the most remote corner of Somalia.
From this point Italy could have retained a democratic, even if militaristic and right-leaning, goverment.