In 1700-1900 England, how were 'manners an end in themselves and not merely a means to achieve propriety by conformity'?

Very difficult to write a good answer; the source is an opinion, and it is difficult to cite sources to support someone else's opinion.

That said, let's break phrase 1 down.

Manners were an end in themselves and not merely a means to achieve propriety by conformity.

Implicit in the second half of the sentence is the assumption that the reasons we display manners is to achieve propriety through conformity; this is a legitimate assumption - if you google "propriety, conformity, manners", dictionary.com will explain that propriety is, "conformity to established standards of good or proper behavior or manners." We practice etiquette/manners in order to demonstrate good behavior; whatever standard we all conform to is the standard of good behavior because conformity is good and diversity is a form of deviant behavior, and therefore bad. (Sorry, the implicit sarcasm in the source material makes it difficult for me to constrain my sarcasm). According to society, we practice manners in order to achieve an objective - to demonstrate that we are members of society, that we respect other members of society. We demonstrate our respect for society by conforming to social standards.

Lord Chesterfield however is proposing that manners serve a different function. Manners are not a mean to an end, they are an end in and of themselves. Manners are like art - artists create art because they want to create art; dancers dance, singers sign, writers write, etc. not because they want the worshipful admiration of their peers, not because they want awards, but because they have a creative energy that they must exercise. Or, to put it another way, I practice martial arts for the simple joy of it. I have no expectation or hope of ever using my martial arts; I don't do it in order to....., I do it because I like doing it. Lord Chesterfield is advising his son to pursue manners because manners are intrinsically worthwhile, individually satisfying and good.

My ex-wife used to say that manners are even more important among families; our lives are full of so many opportunities for conflict that it is vital we use manners to defuse those conflicts and prevent them from festering. She was taking the opposite position of Chesterfield - she was arguing that manners have a purpose, and objective.

I'm stretching even further on #2 - If we assume that manners are like artistic expression, and should be pursued even in the absence of any benefit, then it is not unreasonable that language should be treated similarly. Language is not a technique for communication, but an artistic medium. Elegance of language is a goal in an of itself. Like poetry or modern obfuscated code, we should respect the skill and effort involved in crafting language, even if the code or language is of no value. "The studied perfection of this language falls unseasonably on modern ears. " - the modern ear expects language to be a tool of communication; we distrust speech that requires the listener to strive to decode the craft of construction. Speech in Lord Chesterfield's era included more clauses, more techniques, more quotations, more references, more allusions to shared concepts (conformity and propriety). It was less effective at communication of simple concepts. (Was it Feynman who blamed the O ring disaster on powerpoint? The argument was not dissimilar; people wanted pretty art filled powerpoint slides; the slides confused the simple message, "This is dangerous".)

I cannot conclusively say what QC Du Cann is arguing, but that's the meaning I extract from the phrases above.