The older Merton son is already married, and to a character a good deal more unpleasant then Miss Bunting. (Amelia must have been great fun to play for the actress!).
What I would have loved to have seen would have been Miss Bunting (shallow spouter of party propaganda, very few real insights, claims to represent the workers though she is clearly from a fairly affluent farmer's background or similar- that car was not paid for out of a schoolteacher's wages!) pitted in argument against Thomas (clever, well informed, dry wit- and the genuine working class article!). Unlike Daisy he would not take kindly to being patronised by somebody from a different class who generously undertakes to think for him.
I don't think she is bitter at all: I think she is a spoiled child who has always been told how clever she is. It is easy to forget that the only class division is not between the Lord Granthams and the servants and get the impression that everybody who is not an earl is an oppressed member of the proletariat. In actual fact, the class difference between somebody like Carson or Miss Bunting otoh and Daisy on the other is probably at least as great as that between Carson and his employers. From Daisy's place in the world, Miss Bunting is also one of the privileged few- only Daisy is so starry-eyed she cannot see that.
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