I once read a critical essay on S&S that points out that in many novels the hero/heroine is defined as in conflict with society, and all its stupidities and hypocrises, and the problem for the hero/heroine is how to escape (and often such central figures fail and die, eg Madame Bovary or make unhappy marriages, eg Portrait of a Lady), while JA is concerned about how to live in society, including all its stupid, uncaring people, and perhaps find some measure of happiness.

Reading S&S with this in mind, I notice many parts where the rules of society make life more tolerable for Elinor, and for those around her, while Marianne both suffers and causes suffering more than she ought.

The highest example of the virtues of society is in Lady Middleton's behaviour:
Lady Middleton, though in the middle of a rubber, on being informed that Marianne was unwell, was too polite to object for a moment to her wish of going away, and making over her cards to a friend, they departed as soon as the carriage could be found. (chpt 28). Lady Middleton gives every sign of caring very little for Marianne as a person, but manners oblige her to provide the only assistance that Marianne can benefit from.

Meanwhile Mrs Jennings and Sir John, for all their good-heartedness, cause a lot of agony to Elinor and Marianne. Doesn't the line The letter F -- had been likewise invariably brought forward, and found productive of such countless jokes, that its character as the wittiest letter in the alphabet had been long established with Elinor. (chpt 21) speak of excurciating suffering for Elinor?

Of course JA shows us such characters as Colonel Brandon and Elinor, who combine attention to the forms of politeness with real feeling. There is, in JA's world, no dichotomy between politeness and genuine kindness, both can be together in the same person.

The Romantics do not appear to have a solution to how to live in a world where there are people like Lady Middleton or Sir John and Mrs Jennings. I cannot take Rousseau seriously as a critic of society - he let his mistress abandon their children at a foundling hospital as each was born. Perhaps Marianne can be seen as a weaker version of Rousseau, she also neglects her responsibilities to others (though in a much milder way).

Another problem with Romanticism I think plays out in Marianne's story with Willoughby. It has no way to cope with decit and sheer nasty behaviour. Elinor is protected somewhat - she doubts Edward's intentions to her (chpt 4), she is used to struggle and self-command so she gains the trivial, but not meaningless pleasure, of not showing how harmed she is by Lucy's revelation and conversation. Marianne has gone further, pinned her hopes more deeply, and when betrayed has no internal resources to fall back on and must depend on her sister - and Lady Middleton's courtesy.

As you say, the formal rules of society might have protected Marianne more - by raising a doubt about Willoughby's intentions when he failed to propose. Instead she defies proprietary and social actions, because he appears to share her true feelings. And she continues to deny her doubts as long as possible, making his eventual unmistakable betrayals perhaps that much harder.

JA portrays the dangers of abandoning the habits, values, and rules imposed by society.