"The origins of the latest epithet in vogue are harmless." The origin of the word is irrelevant. Words mean things and can be harmful, regardless of the origin of the word. The meaning and context of words can change over time, regardless of the origin of the word.
Bringing Charles Darwin into the conversation does not help your point.
"Is it worse to have some condition of your birth used as a casual insult -- a reminder of your misfortune? Or is it worse to be constantly patronized, often behind your back by throngs offended on your behalf?"
This is a false trade-off. The whole conversation started because someone used a harmful word, knowing full well it was harmful. If they refrained from using the offensive word they knew was offensive neither condition would have happened (casual insult or patronization).
Are the terms "bald," "stupid," "fat," "anemic," or "impotent" harmful when used abstractly and negatively?
Come now. At the root of all of this is an ableist, patronizing assumption: People with mental disabilities must have the language used around them carefully policed because they can't handle the implied disfavor and emotional harm that language may communicate via their own agency, not like the rest of us.
Bringing Charles Darwin into the conversation does not help your point.
"Is it worse to have some condition of your birth used as a casual insult -- a reminder of your misfortune? Or is it worse to be constantly patronized, often behind your back by throngs offended on your behalf?"
This is a false trade-off. The whole conversation started because someone used a harmful word, knowing full well it was harmful. If they refrained from using the offensive word they knew was offensive neither condition would have happened (casual insult or patronization).