] ' " No Marieanne never... I am guilty, I confess, of having wished you to treat our acquitance in general with greater attention; but when have I advised you to adopt their sentiments or confirm to their judgement in serious matters ?"

To me, this quote is at the crux of the divergent behaviours of the two sisters.
Marieanne embodies the question of the C18th Romanticists; Why deny one's true nature or feelings to fit into society ?
Elinor veers on the side of society and civilty or 'tolerable politeness', and this is what she desires for her sister.
My impression from the quote is Elinor doesn't belittle Marieanne's own abilites. She does not expect Marieanne to deny foolishness or vulgarity in people when she finds them, but to learn 'to behave with tolerable politeness.'

Yet Marieanne hasn't learnt patience with her acquitance in these chapters. Her behaviour remains self-centred, Elinor realizes Mrs Jennings is a means to get to London and Willoughby. Sensibility is a road paved wit selfish intentions ?

The narrational judegement appears to be that Marieanne needs to mature whilst, in her essential behaviour, Elinor does not.
Instead, Elinor appears to go through alot of testing.
She is truly tested at this time. She gives way to pressure from her mother and Marieanne so agrees to go to London.
Elinor is also being tested severely by Lucy's disclosure.
And now, she must try and avoid Edward in London.

For myself, I'm less less sure on your thoughts re: John and Fanny. :)
Their failure to meet Henry Dashwood's dying request to provide for the Dashwood ladies in any way, in my view, John does not live up to his rank as head of their family. He abrogates his responsibilities as their brother and protector.
I agree they should be civil in conduct and pay attention toward John and Fanny- a test for both sisters.
However, I see respect as the flip side of responsibility so I feel less sure about respecting John.

Otherwise, I agree your've done another interesting post on sisters, great analysis ! :)