I don't know if my post is sufficient to answer your post, and I admit it may be a bit incoherent. But these are my ramblings that were inspired by your post.
I think you bring up some good points and I think a lot of criticisms fall into those traps you presented.
Some would question the fact that many of the films nominated for Oscars past and present (since they started becoming a bit more inclusive with indie films) have broad appeal as many of them don't make tons of money (though some do and there is a bit of a box office boost for getting award attention). People also complain that movies that win Oscars aren't relevant to the tastes of the broad masses. The term "Oscar movie" is actually used as a derogatory to make fun of how out-of-touch those films are to the taste of the masses. Many of the indie and/or non-nominated non-indie films do show technical virtuosity as well. A lot of nominated films in the past and present may not show as much technical virtuosity as films not nominated, indie or not. I guess my question becomes why does diversity within film have to be relegated to the indie sector as opposed to being created within studios? Why are they "niche?" And why should be satisfied with that being the answer and should only accept it because of conventional wisdom? Some question the age old wisdom that the only movies making money feature whites and only whites have "broad appeal". For a lot of people, it just seems odd in this day and age and it doesn't pass the smell test.
As to why only these films were seen worthy to be nominated goes into pretty complicated questions of who makes up the members of the Academy passing off judgment of artistic merit, who dictates the standards of good and high art/taste (Lupita Nyongo's point), who is in charge of the studios who are buying distribution rights or producing films and how do they choose which ones to campaign for and promote during award season, and even if we accept its based on merit, why is it so hard for films that aren't so status-quo to be recognized when the Academy hasn't let lack of box office or media attention or even lack of spectacle or excellent craftsmanship stop them from nominating very small films before.
As Spike Lee said, the Oscars aren't the real battlefield. But I think people latch on to it because as much as people mock it and disagree with it, it still means something to a lot of people and the Academy and Hollywood puts a lot of stock into the awards and promotes it as meaning more than just giving a trophy away but as a message of this is what should be seen as the pinnacle of our industry.
As for film being bigger than Hollywood. True. But as you said, it's an American industry and Oscars award American films for the most part. Many who are voicing their disappointments are American movie goers and are seeing this through the lens of Americans and its issues with representation and lack of it. Many of them are just regular people who don't pay attention to film festivals either. But they want to see a mainstream industry and a mainstream award show start having, producing, promoting, and awarding creations that represent the moviegoers who are going to see the movies. Hollywood is taken to task because that's the industry and they have the most influence and therefore can make the biggest difference or are the biggest influences in perpetuating the same cycle. It's like calling out big corporations for lack of diversity in employment and in management positions even though there are smaller businesses where they don't as much of a problem. Sure an industry is defined more than just this big corps, it's still a big deal as they are seen as the industry.
As to which specific actors, actresses, writers, producers, and directors, I guess it's just odd to only see mostly whites being honored and represented and that they, for the most part, are the only ones producing films or giving performances worth giving Oscars to. Just a few names I remember reading about on Oscar-related blogs before the nominations came out:
Michael B. Jordan won the prestigious National Society of Film Critics Award for Best Actor in Creed
Idris Elba was nominated for a BAFTA, Golden Globe, and SAG and won the Washington D.C. Critics for Best Supporting Actor in Beasts of No Nation.
Abraham Attah received notices for the same film.
Cary Jo Fukunaga, director of Beasts of No Nation.
Trangender actresses of color Kiki Kitana Rodriguez and Mya Taylor received positive notices from critics associations for Tangerine as did the film itself.
Straight Outta Compton received both SAG and PGA nominations which a combo of both usually leads to a Best Pic nod.
RJ Cyler received a Critics Choice nomination for Me and Earl and the Dying Girl.
Shameik Moore in Dope received the same nomination.
Oscar Isaac in Ex Machina received a few nominations for his supporting role.
I'm sure those who have their fingers on the pulse of current films could identify more examples of quality films that were at least as acclaimed as a few of the nominated films and actors, but haven't been given the attention and serious campaign backing. Not saying any of the listed above deserved Oscar nods over others, as that's a matter for debate. It's just that there choices and its odd that actors who were considered just as much as on the fringe to be nominated as those who ended up getting nominated don't get nominated and have something in common with one another that nominated ones do not share. But then, why are these fringe picks? If the list isn't that exhaustive, the question goes back to why aren't there more choices to choose from that are considered Oscar worthy? What is Oscar worthy? It becomes circular.