In a surviving Fascist Italy you could have Eritrea remain in Italian control for longer. Getting the demographics in Italian favor will be very difficult, particularly when the colony has to "compete" with Libya, Somalia, and Ehtiopia for settlers. Libya, being now a part of "Metropolitan" Italy thanks to Balbo, will get the most the soonest.
Instead I'd see it like this:
Italy Neutral in WW2.
After the war, oil found in Libya gives Italy the raw cash they need to keep up their wasteful, mismanaged colonization efforts.
Money is sunk into the colonies (roads, ports, schools, buildings). Settlers continue to arrive, though never break 20% in the horn. Lives are lost fighting insurgencies in Ethiopia, primarily Eritrean, Libyan, and Somali lives.
Despite official rules against "fraternizing" with the locals, Facetta Nera remains a popular song for a reason. A growing mullatto class expands in the Horn, becoming a de facto middle class of merchants.
Over the next generation Ethiopia proves an expensive quagmire and the Amharan heartland and west are "given" independence as a rump Ethiopia. Ethiopian Italians, Ethiopian collaborators, and the growing Ethiopian Mullatto class move to Somalia and Eritrea.
As the 80s go by the exterior horn is indellibly linked economically to Italy, but nationalist and ethnic pressures remain. Demographically very diverse, but with Italian as the lingua franca and the various African and mixed groups as much at odds with each other as the Italians.
Doubtful it'll be enough to merge the region into "Italy" as full provinces...they're having a hard enough time with that in Libya where they're actually approaching ethnic parity with the locals at the coast.
Instead eventually Eritrea and Somalia gain what we might call "Union" status, and eventually are part of...call it an Italian "Commonwealth".