I went ahead and looked up David Clay Large, and his faculty page lists his academic focus as Modern Europe and Germany. Since he is not an Ethiopianist, it immediately raises warning signals about any claims he makes about what the Amharic elite did or did not consider themselves.

Personally, I don't think an idea of being "caucasian" was internalized among the Amharic elite by the 1930s. The theory of racial categories of Caucasian, Negroid, Mongoloid, Australoid peoples, was developed in the mid-19th century in Europe and North America; and it is possible that such racial theories could have been introduced to Ethiopia through diplomatic contact or European missionaries. However, I don't know of a good reason why Amharic elites would quickly incorporate it as a part of their self-conception of identity.

However, the idea of Amharic elites identifying as "caucasian" and using that to justify holding slaves rings false. Such an argument would run counter to the general trend of Western powers abolishing the practice of slavery.

No, I think the justification of slavery in Ethiopia at the time was a case of it being a long-standing and pervasive social institution.

Additionally, Haile Selassie's predecessors had not sought to justify slavery, but since the reign of Menelik II, had made various treaties and laws that sought to end the practice of slavery in Ethiopia. These laws were usually ineffective because of the perception that slavery was a social institution, but also because there were concerns that should slaves be emancipated they would become an underclass with no means of supporting themselves.

this Library of Congress blog post

as well as this follow-up

give a well-sourced explanation of the period from 1870-1942, and the efforts of various emperors to stamp out the practice of slavery in Ethiopia.