@Celebrim, please try to read my words without ascribing motives to me.
If you didn't want to be perceived as equating orcs with black people, perhaps you shouldn't have brought up "the curse of Ham". So what is this, the clown nose off, clown nose on defense? You decided that a religious reference used in defense of enslaving African people was so relevant to this discussion that it could be introduced in a defensible manner. Now you are trying to pretend you didn't do that?
Right. But half-cows, half-swans and half-trees aren't hardwired into the game. Half-demons and half-dragons are marginal, at best.
Tieflings, dragonborn, etc. To a certain extent, nothing is "hardwired into the game". Orcs haven't existed at my table since 1989, when I made the decision to drop orcs from the game entirely because they were redundant with the more interesting goblin-kind (the three caste species of goblins, hobgoblins, and bugbears) and I didn't need to type of monstrous humanoids that were predisposed to evil in one game world. How much something is a part of a particular game is up to the DM.
Orcs are not real. They are a function of our imagination.
Yes, you should keep that in mind. The fact that orcs are not real means it is very difficult to assert anything about them as a factual statement. So much of what you are claiming about orcs is not based on lore, but based on your choices about how to see orcs. So yes, it is a valid statement to say that if you make orcs basically human but with a different appearance, then they probably are basically human in violition, rights, and dignity. But this conception of orcs is just one of many, and it tends to be a recent one - I'd say one that was mostly post World of Warcraft when Orcs were made an equal playable race to humans, and thus needed very much to have a backstory with equal violition, rights, and dignity. But because orcs are not real, you can't say that that is the one right and true way to imagine orcs.
(Moreover, that tendency to see orcs as a metaphor for human tribal people groups is I think an incidental and somewhat unfortunate side effect of the World of Warcraft presentation as well.)
Where have I evinced any opinion about alignment?
By saying that "There is nothing in any version of the rules to suggest that they are automata, incapable of choice." you are evincing an opinion about alignment. If something is always a particular alignment, then at least with respect to moral choice, they have no real volition. It's only recently that we're seeing orcs presented in a way that suggests that they might only be usually evil, and so have some choice in the matter. But the classic "are orc babies evil?" argument predates the "usually" and "often" categorization, nor does even the "often" categorization preclude concluding that orcs are divided in alignment among LE, NE, and CE as a way to make later editions backwards compatible with different tables answers to that question.
The mythic particulars describing the aetiology of orcs are unimportant.
The particulars of the aetiology of any living thing are absolutely important to determining its rights. How and why something came into being is probably the most important determination of how to classify something and determine its rights and dignities. Between aetiology and sentience, there aren't a lot of obvious things that determine personhood, or the nature of personhood. (If you'd like to claim aetiology doesn't matter, only sentience, then I have some hard science fiction examples for you.)
The fact that their status is rooted in some mythic event, or has an irremediable divine cause, is the point.
What point? Even in the real world, it's not unusual to see the whole of humanity has having an "irremediable divine cause". For example, the Declaration of Independence famously declares, "We hold these truths to be self-evident, that all men are created equal, and are endowed by their Creator with certain inalienable rights". So yes, the creation of something matters, or at least is widely perceived to matter.