1. Why was it left alone by the imperialists. Was it because it would be politically incorrect to subjugate a fellow Christian nation?
Added to the other traditional "imperialists", it's also probably worth noting that Egyptian Khedivate (a wannabe imperialist, at the very least) was a rather frequent invader of Ethiopia and started about 30 years before everybody else in the 1820s, as part of its quest to control the Nile and secure sources for slaves.
I'm not sure about the "buffer state" reason for Ethiopia's survival, given that the 1870s Egyptian attempt and the 1900s Italian attempt pretty much aimed for annexation and yet few countries actually seemed to do much intervening. Ethiopia shouldn't be important for protecting European colonies on the Red Sea littoral, especially as the interior of said colonies were pretty much independent states in their own right (i.e. the Dervish State in British Somaliland). So I wouldn't say that the Siam-analogue works.
I'd argue that Ethiopia survived because for most of the 19thC, inter-European competition in the Horn of Africa was at a lower level than in the rest of Africa. Competing British and French interests were the driving force behind the death of most West African states, while Portuguese/Belgian/British competition did so for the south. There just wasn't that similar level of competition throughout most of the 19th Century in the Horn of Africa, partly due first to Ottoman-Egyptian and then Anglo-Egyptian dominance. There wasn't any need to destroy Ethiopia because for most of the century, there was no risk of Ethiopia falling sway to a potential competitor.
This situation began to break down by the end of the Century, with Italo-British competition, but that was stymied by the Italian defeat at Adwa. And before anybody points it out - Fashoda. This indeed was a competitive situation, but it was aborted early (at the "competing expeditions" stages) before it could affect Ethiopian independence; after that France never really challenged British imperatives in the region. I'm pretty certain that if the French got Fashoda, Ethiopia would have been colonized, or more likely partitioned, before long.