So now Harriet has had an offer of marriage, which Mr. Knightley views as quite promising and which Emma of course persuades Harriet to reject. Emma seems to think poor Robert Martin is social climbing in wooing Harriet, even though, as Knightley points out, Harriet was quite happy in her station in life until Emma began putting ideas into her head.

IMHO, it's interesting that while Emma says she could not be interested in the Martins and could not visit Harriet were she to marry Robert, Knightley, who is definitely of the same social class and seems to be more land-rich than the Woodhouses, is on friendly enough terms with Robert for Robert to confide in him about his plan to propose. I suspect K. isn't always snobbish enough for Emma's taste.
Harriet now has a room of her own at Hartfield, so whoever is paying for her to be a parlor boarder at school isn't quite getting his money's worth, is he? Emma assumes Harriet's father to be a gentleman, probably with money, but it seems unfair to make him waste his money on room and board that isn't being used.
And how does K. guess that Emma is trying to match her friend with Elton? Is it that Elton is the only man in town of a sufficient social rank, or that he's the town's most eligible bachelor? In any case, I like K.'s warning that when the ladies are not present, Elton talks of his future plans in a way that indicates he's not going to waste his good looks. So he too is looking to rise in the world, and if that's the case, he won't want to marry someone as poor as Harriet.
Given that warning, it seems almost cruel on Emma's part to keep fanning the flames of Harriet's fancy, except that Emma sees only what she wants to see.
In Ch. 11, we meet the John Knightleys, and we get an idea of how big a house Hartfield is, when we learn that the whole family -- five children plus "a competent number of nursery-maids" -- have come for Christmas. Even if someone uses the room that's been set aside for Harriet, that's a lot of extra people to house. It must be very lonely indeed for Emma now that she's sharing this huge house with only her father.