Four, and most important, the in-game fiction, whatever that is (species? magical creation?), is not really what we are talking about. The idea that the different societies and peoples in the real world are not actually or fully human was and is a core fantasy of the racist worldview. It's this dehumanization that allowed for and justified so much violence. So I don't think it's a good idea for an author or in this case game designers to uncritically replicate this dynamic and especially the specific language of that dehumanization (as it has been used historically) when creating a fantasy world.
Not to gloss over the rest of your post, but I wanted to speak to this bit.
I absolutely agree that other societies being inhuman, or perhaps subhuman is a better way of putting it, is a huge part of racism. And I am coming from the perspective of a white male in the US, so my perspective is, by its nature, limited. I try to remain keenly aware of it in these discussions.
Nonetheless- at a certain point, we have to choose one: dwarves, elves, et al, are just funny looking humans; or, fundamentally, they are
not human.
"Not human" needn't mean
lesser in any way, though. It just means
different. And while, for example, an elf might be "lesser" in sturdiness and endurance by virtue of its -2 Con score, that doesn't mean it is "lesser" in terms of its value as a living being. Which is what the racist trope/ideology is about.
And while I agree that it's good to avoid the language used in perpetrating the racist world view, that doesn't mean it's impossible to describe a different, definitely non-human, race/species/whatever term you like in a way that sets them up as absolutely nonhuman without making them seem like lesser beings.
And then there are monsters. D&D is a game, fundamentally, about fighting monsters. The humanization of everything gets ridiculous in this context- I mean, at a certain point, we end up with people arguing about how it's unfair that we depict redcaps as universally murderous when they are pretty much murder incarnate in the lore. We'd be better off (in my opinion) leaning in, in this case, to the inhumanity of redcaps. "They're basically murder elementals", said someone in that thread, and that's how I see them- not as something to be humanized and sympathized with, but as unrepentant, irredeemable evil.
Anyway, nonhuman need not mean anything in terms of whether a creature is "lesser" or "greater" than humanity. Just that it's different. And while we should be careful to avoid perpetuating racist tropes, we should also be careful not to project too much humanity on all the monsters in the game. Especially those that aren't humanoid as a type (and no, the common definition of humanoid-as-shape doesn't matter a dingo's kidneys here).