I have put together this attempt at an answer which, albeit quite long, is far from the last word on the matter (and indeed I already feel like I should revise it), since various authors have approached the issue from different angles resulting in slightly (or, in the context of historiography, perhaps vastly) different interpretations.

In general though, the "coup" narrative is regarded at best as an oversimplification, since - while the contents and character of fascist violence during 1920-22 may appear manifest to us, and therefore the King's choice does appear to depart from the true spirit, if not from the letter, of a supposedly liberal constitution - Mussolini's appointment as Prime Minister took place in a legal fashion and with a significant degree of political and popular support.