Slavery was abolished throughout the British Empire in 1834.
Slavery was illegal in Britain from an even earlier time, the 1770s or so.
There was nothing anachronistic about Mill's questions.
Moreover, you really miss the point here. There are differences between present and past attitudes to be sure. But the differences within an epoch can be quite great indeed.
What is "our age's" attitude towards your beloved question of transsexuality? The answer is that there isn't any one attitude characteristic of our age. There is a multiplicity of opinions.
So it was with slavery. We can say that expecting 19th century people to take 20th century attitudes to racial equality is anachronistic. Very few people believed in racial equality in those days.
But that doesn't mean that when it comes to slavery we make the slaveholders' view the only characteristic view, normative of its day. Not everybody was a slaveowner. Not everybody believed slavery was right.
Very many people believed that slavery was wrong (in various ways and with varying degrees of conviction). The way people thought in Kensington or Westminster wasn't any less characteristic of the age than the way they thought in Georgia or Alabama.
Mill certain was "ahead of his time" when it came to expressing a believe that colored peoples should have a say in the government that they lived under. I wonder if he'd have shown such consideration to the Indians (or the Irish), but the fact that he could ask the question about the consent of the slaves and publish it meant that the question was conceivable and "askable" in his day.
When people start to justify the Confederacy sooner or later, they often end up minimizing and dismissing the issue of slavery. Sooner or later, you put yourself into the position of the slavemaster. When I was in high school and thought like you did, it was much the same with me. I got over that in time. I hope you will too. It's really too much to be locked into such a way of thinking (or not thinking).
Good post but pearls before swine and all that. He’s invested far too much in his elaborate house of cards to dismiss it now. He epitomizes “Lost Causer” with an emphasis on lost.