I would say that this type of argument could be committing the fallacy of reversing cause and effect (also known as the "questionable cause fallacy"). In these examples, a criticism is made of some aspect of a narrative work. The defence then points to a proximate cause for that aspect of the work, where the proximate cause is itself another part of the narrative work that is caused by choices of the writer of that work. The proximate cause is still a decision made by the writer of the work, so it is actually an effect of the ultimate cause (the decisions of the writer) rather than a proper cause per se.
You need to be careful not to jump to the conclusion of fallacious reasoning if this is not justified. There may be cases where the proximate cause appealed to in the defence of the criticism is itself a reasonable writing choice in the narrative work at issue. The point here is that the proximate cause is also an effect, so it also needs to be defended on its own terms. The defence of pointing to this proximate cause just pushes the criticism back a step, so that the criticism now becomes an implicit criticism of that preliminary aspect of the narrative. Maybe there is also a good defence of this narrative choice and maybe there isn't, but the argument then needs to move to that step.
Here is how this issue plays out in the examples you give in your post:
Example 1 (sci-fi movie with one ethnicity of actors): A sci-fi movie is criticised because every role is played by people of the same ethnicity. The defender explains that this is realistic because there is only one race on the planet where the movie takes place. In a certain respect this is a perfectly reasonable response - if we take the characteristics of the planet in the story as fixed by this constraint, then clearly it is appropriately realistic for the actors to be of an ethnicity that matches this requirement of the story.
Here we have a situation where the writer has chosen to construct a story about a planet that only has one race, and as a result of this choice, it is then rational for all the acting roles to be played by people who resemble that race. Here the race of the actors is the effect complained of, the fact that the planet has only one race is the proximate cause of this, and the fact that the writer has chosen for the story to take place on such a planet is the ultimate cause.
The reason that the defence is unsatisfying in this case is because the proximate cause used as a justification is an effect, not a real cause. After all, unless there is some good reason to the contrary, the writer could have set the movie on a planet of people with multiple races, which would then have allowed the casting of actors of different ethnicities. So now we need to go back a step and argue over whether there is any good reason for the planet in the story to have this characteristic. The initial criticism might then become an implicit criticism of this preliminary narrative choice, and the argument moves back a step.
Example 2 (protagonist with extreme ideology): A movie is criticised for having a protagonist who supports some extreme ideology. The defender explains that this is realistic because the back-story of the character.
Here we have a situation where the writer has chosen to construct a story about a protagonist with a particular back-story, and as a result of this choice, it is then reasonable (in a causal sense) for the character to hold this extreme ideology. Here the ideology of the protagonist is the effect complained of, the back-story of the character is the proximate cause of this, and the fact that the writer has chosen to make a story with this kind of character as the protagonist is the ultimate cause.
In this case, what we really have is a complaint that a certain type of character is featured as a protagonist in a movie. It is a stretch to criticise story-telling about a certain type of character, so really, the criticism is not about the characteristics of the character, so much as the fact that he is featured in a primary role (as the protagonist). The argument should really now move to an explicit argument about whether it is reasonable to feature this kind of character as a protagonist in a story.
Example 3 (war movie): A war movie is criticised because it depicts allegedly fictitious countries that resemble real-world countries, and it depicts one of these in harsh terms. The defender says that the antagonist country in the movie is an fictitious altered-version of the real-life country, which has certain additional characteristics that make the it justifiable to vilify that country.
Here we have a situation where the writer has chosen to construct a story about a fictitious country which is (allegedly) an altered version of a real-life country, and as a result of this choice, it is then reasonable to vilify that (allegedly fictitious) version of that country. Here the vilification is the effect complained of, the fictionalised changes to the real-life country is the proximate cause of this, and the fact that the writer has chosen to use a country that is a close analogy to a real country is the ultimate cause.
Unlike the previous scenarios, this one really hinges to a large extent on the honesty of assertions of what is intended to be fictitious, and what is intended to be a true analogy and implicit vilification of a real country. In any case, the argument should again take a step back, since it is really an argument about whether it is reasonable to depict a fictionalised version of a real-life country, and then vilify this country on the basis of fictionalised variations (or contrarily, whether this is just some intellectually dishonest propaganda about the real country).
