An author can absolutely tack things onto any narrative they choose to write. An author doesn't get to claim something is built-in just because they happened to write it that way. To me, "tacked-on" vs. "built-in" are indicators of quality of the integration (of the thing we're discussing).
Totally agree with that, but you'll notice that if an author integrates something badly, it's considered a flaw.
And notice that by essence, altering a work with political intent is tacked-on. You can't integrate it without changing the work, unless it was already in the work to begin with (in which case you don't need to add it).
You kind of pointed precisely one of the main reason I despise shoehorning political stuff into existing work (the second being the nature of altering a work for political purpose, which you refer later as my problem with the "intent").
But if someone merely cast the immortal demigod as a black guy, even if it were for the sole reason of better on-screen representation, I wouldn't consider that tacked-on in the slightest.
And yet is is, and you actually admits it low-key later by recognizing that Gandalf is "implicitely" white. More about that later.
I consider representation in casting to be a fair reason to cast people in a specific role. This isn't me saying "find someone who can't act", obviously, they have to be able to act, and so on.
Representation in casting is a political issue, it has absolutely no place in artistic decisions (unless the work is itself about said political issue). Arts are welcome to address politics, but politics should stay away from art. Attempts to modify art because of political agenda has a name : propaganda. And it's not something I want to condone, and people condoning it are typically hypocrites because they only condone it as long as it goes their way and are quick to denounce it when it goes the other (see all the - valid - complaints about whitewashing and compare them to this thread).
Also, why would "being able to act" is acceptable but "not looking like the character is supposed to" isn't ? After all, both are about adding immersion to the work (I'll recognize that good acting is generally more important than appearance, but both counts nevertheless, and if you accept one on this ground, why would you reject the other ? Screams of double-standard to me).
What artistic integrity is present in making Gandalf white? What value does this give the setting?
I already answered this point several times, it's about the general tone and flow. It's even been the focus of several posts, I'm just a bit tired of repeating myself here.
Would you agree that the reasons for making a change, and the impact of the change itself, are two separate things? i.e. one could be bad, but the other could be good? Or both could be bad, both could be good, etc? Because that's how I see it. It seems to me that your perception of the reason colours the impact of the change, even when the resultant quality might not actually suffer.
Intent and results are of course not the same (though they aren't completely separate, because the reason why some change are made is often strongly correlated with the quality results you get).
But the main point is that your definition of "quality doesn't suffer" is glossing over some pretty impactful parts - see below.
This is what we keep coming back to with the hypothetical of Gandalf being black. I've demonstrated that there is no qualitative impact on the actual plot and character interactions
Wrong, you've
claimed it, you've not
demonstrated it. If you gave him blue skin and flappable ears and didn't change a line of dialogue, it would also not change anything in the story. But it would look out of place, and the fact that nobody would react to it would make it even more weird, it would not "demonstrate" that it made no qualitative impact on the work*.
To come back at the previous "tacked on" and "setting tone" parts above, lots of things are implied and not detailed, the implicit being one of the most powerful tool in storytelling (elegance in writing is often about making the reader understand what is conveyed implicitely rather than explicitely). The fact something is "implied" doesn't mean it's "ignorable" nor that it doesn't participate in the overall feeling (on the contrary).
It's implicit that Gondor is "medieval european" in feel, even if not everything is detailed. When thinking about the soldier of Gondor, we imagine typical european gear. If you had suddenly a few guys in samurai armor, it would be pretty noticeable. That would either be deliberate (implying there are other cultures inspired by Japan in some other place of the world and they are somewhat involved in Gondor, or that there are mercenaries/travelers from afar, whatever), in which case it can participates to the worldbuilding and feel if it's well integrated and written, or just
tacked on, a flaw that stick out and breaks the flow and blurry the perception of the setting. Forced diversity falls square in the latter.
* I should normally not need to point it, but just in case : Gandalf with blue skin and flappable ears is obviously not on the same level as him being black, the point is precisely to push the concept farther to illustrate it, it's a common reasoning device, etc.