If HS (Haile Selassie) decides Fascism is the way to go, I cannot see him going the Italian route. AFAIK he was a devout Coptic Christian, so the Italian way wont work. If he tours the nations of Europe, and visits Portugal, Italy and Greece, his version of Fascism will be a mixture of Greek and Portugal, and possibly later Spanish influences. I expect a civil war to occur, between his govt and the Muslim citizens. There would be forced expulsions at the least. If they convert, fine. If not, they have a lesser status or are expelled. This will very possibly cause difficulties with nearby colonial states, but it will happen. Only Italy may find things burdensome enough to rattle sabers, and if this civil war happens early enough, they wont attack for many years. In the interim, banditry and a low grade guerilla war will occur.
What he can do to ameliorate the outcries is to outlaw slavery in the kingdom. Slavery was used by the Italians as a justification for invasion later, if he takes this action early (he may well do this and simply enact some form of legal indenturement to satisfy both sides) then one weapon is taken from the Italians and some public support is gained at home. If he blames the Muslims for continuing the practice and uses it as justification for his program against them and unification of the kingdom then it will silence much of the foreign outcry. The 1920's are not the same as 2018.
IOTL there was an oil lease between the US Standard Oil Company and Ethiopia. Unfortunately that was in 1935, and its implementation was too late for the US Govt. to allow it to proceed, the US Govt. not wanting to anger some of the business community and the many voting citizens of Italian descent. I am not sure when Standard Oil discovered oil in Ethiopia, but if its a few years before 1935, and they get stuck in there, then Ethiopia has both more income AND a sizable portion of people in the US and other Colonial powers who will be vocal about keeping the nations independent. Money talks, and if Standard Oil stands to possibly loose money, the US may enact sanctions all on its own.
Not having a crystal ball of foresight for possible AH outcomes, just how more foreign involvement in Ethiopia will turn out is hard to say. Does it cause Mussolini to turn more towards Albania, or to manufacturing a casus belli with Yugoslavia? Might it drive him to continue the Stresa Front in an effort to gain fame and notoriety? I don't know, but tend to believe it possible. It is also possible (simply based on being another Fascist state) that the Fascist countries (Italy if it doesn't try and invade Ethiopia) will form some type of diplomatic organization to further trade between themselves. Past this, I have no idea. My prognostications are done.
