Between the three main reasons, I would see C) as most decisive.
The religious factor A) is sort of odd, but I guess true. Doctrinally, the Ethiopian Coptic and Russian Orthodox churches have a different rite and christology, but I guess at some point, I wonder when, there was some sort of agreement they had something in common (kind of like Papal acceptance of Maronites into the RC communion).
While obviously that was not going to be a compelling rationale in the officially atheist USSR, there would have been plenty of ideological justification for support of Abyssinia: 1) By this point in time (1935) the USSR was endorsing Collective Security against aggression. It sent volunteers to Spain a year later to demonstrate commitment to that point. 2) ...and of course anti-imperialism, support of national independence, even if under a non-communist (like Nationalist China or Turkey) or even monarchical (Afghanistan, Iran) native regime).
The decision to do nothing in Abyssinia while doing something in Spain still requires a little explanation though.
I can see a plausible Marxist argument for (B) a feeling that the Haile Selassie feudal regime was more socially regressive than even Italian colonialism under dynamic bourgeois-imperialist auspices. However, did any one really articulate that point of view in the Soviet Union or COMINTERN at the time? I sort of doubt it.
That leaves us with (C) Diplomatic caution. Again, two things need explaining.
1) Why would Abyssinia have been more risky for the USSR than intervention in Spain or China? After all, the western powers were protesting and sanctioning the Italians, and this would reinforce their stand. Also, supporting a reactionary monarch in Abyssinia seems politically safer vis-a-vis the west than intervention in Spain. The first is a clear-cut case of opposing aggression and championing national independence, whereas in Spain, the issue of protecting the Republic and opposing the Fascists was more tied up with domestic social revolution, which gave the British and French conservatives the heebie-jeebies.
2) How is the landlocked geography of Abyssinia in the 1930s any different from Russian supported Abyssinia in the 1890s? The answer is it was not. Tsarist Russia's aid to Abyssinia had to pass through Suez to get to the Red Sea, and colonial powers, France and Italy, controlled all Abyssinia's routes to the coast, with Eritrea, Djibouti, Somaliland and Somalia. I don't even know if the Mahdi held any ports in the 1890s, but I doubt Russians transited through Mahdist territory.
So then it seems to me that the decisive element allowing for Russian aid to Abyssinia in the 1890s, but constraining Soviet aid in the 1930s was a different French attitude towards arms deliveries going to Djibouti. In the 1890s, the French encouraged Russian involvement in East Africa. In the 1930s, they probably would have opposed it, most likely because France was trying to win over Italian support in containing Germany at this time. So fundamentally, an advantage Menelik had was that France at the time wanted to frustrate Crispi, while Haile Selassie's disadvantage was France wanted to kiss Mussolini's derriere.
Lastly, Cook brings up the idea of the Soviets not wanting to back a loser. I can see the point, although before the fighting everyone was not sure the Italians would win, or win completely. I would think that with some of the more basic Soviet infantry kit and basic training, Haile Selassie's could have become quite a bit more deadly to the Italians, even if not enough to survive. Also, the situation has almost no chance of escalating to touch Soviet territory, unlike say the China situation.
Would the British really have been so disturbed about arms deliveries that they'd do this:
Worse, since any Soviet armaments ship would have to enter the Red Sea via the Suez Canal, there was the potential that the British would board and search it there, as was their right by treaty, resulting in an even worse international incident guaranteed to outrage the British since it involved the Suez Canal: lifeline to their empire in the east.
All Italian forces were passing through the canal, and the British government and public opinion were very outraged by Italian aggression in Abyssinia, would HMG turned on a dime and become yet more outraged at others trying to help Abyssinia?
Abyssinia was landlocked, so any attempt to ship arms to the Haile Selassie would have needed to go through French or British territory where there was the risk of discovery, which would have resulted in a damaging international incident.
It seems to me that this would have applied with equal force if the Japanese tried to help Abyssinia, as they considered doing, and as others have brought up on the board. Also, I heard Germany sold Ethiopia weapons. Were they ever delivered? If so, what route did they follow and why didn't it cause international outrage?
Thoughts?