A rough idea for a possible timeline: Italy loses the war with the Ottoman Empire in 1911, and doesn't acquire Lybia as a colony. After the earlier defeat in Ethiopia, this prompts the Italian government to concentrate on Eritrea and abandon further colonial conquests. It also prompts very necessary military reforms.
Let's say alt-WWI starts in 1916 after a border skirmish between Austria and Serbia; the tension was there, I'm afraid, even if the murder of Franz Ferdinand is butterflied. Italy still wants its
terre irredente and wants to recover its military lustre, so they join with the alt-Entente and perform marginally better, taking Trieste and managing to hold out against the Austro-German assaults.
The war will have its impact over the Italian people, and there still is chaos and instability after the end of the war. Benito Mussolini is still the charismatic dangerous person that he was, and he remembers well the lesson of the Italo-Turkish war, and of alt-WWI:
don't jump into unnecessary wars for no reason and without due preparation. In 1920 he joins a coup d'etat led by
Gabriele D'Annunzio and is given the Home Office, the Ministry of Colonies, and a few other governmental responsibilities. He decides to plan for a massive expansion of Italian settlement in Eritrea, to relieve what he sees as excess population in the poorer parts of Italy.
In the meantime in Germany a military authoritarian regime is stirring trouble with Poland over the alt-Polish corridor, and attempting unification with what remains of Austria. This triggers a war with a Polish-French alliance, let's say in 1942; Britain sits out but supports the alt-Allies financially, and interdicts trade with Germany.
In Italy D'Annunzio is getting on in years, and leaves much of the affairs of state to his clique, including Mussolini and a small select group. They decide to keep neutral at first, and Italy profits hugely by selling war material and raw resources to both sides; but eventually the desire for greater international prestige, as well as well calculated offers by the alt-Allies and their British backers, convinces Mussolini to enter the war on their side.
Having to fight on three fronts, the Germans waver; they panic and they make the mistake of sinking US ships en route to France. The USA also enters the war, and this triggers the collapse of the three fronts, and a new coup d'etat in Germany. The new German government immediately sues for peace. Alt-WWII is over by 1946.
This leaves Italy slightly richer, and with a more prominent international role. What about the Italian settlers in Eritrea? We'll say that the people that in OTL went to Lybia, about 120,000, went to Eritrea; that the 30,000ish settlers that went to Ethiopia in OTL went there too; and that most of the 20,000 - 25,000 settlers in Somalia went there too. We'll also imagine that Mussolini's colonial policies managed to attract another 80,000 people, partly from Italy, partly from French Tunisia where many Italian colonists settled. This brings up a total of about 250,000 Italian settlers in Eritrea. Wikipedia has a population of 1,141,000 for Eritrea in 1950 in OTL, out of which 75,000 were Italians. Here, we could hazard a guess that we could have about 1,200,000 to 1,300,000 inhabitants, out of which 250,000 Italian.
This is about 19-20%. Possibly not enough to have Eritrea considered a permanent Italian possession. There is going to be something like OTL-decolonization, and there could well be a war for independence in Eritrea.