] Before Elinor even reads the letter, she realizes what it must contain, and her tears are 'scarcely less violent than Marianne's'

] I know Marianne is aware that Elinor feels very deeply and sincerely bad for her, but think that Marianne would be quite amazed to be inside Elinor's head at this time!

I quite agree. If she had been in a more settled state of mind, Marianne might have showered Elinor with praise for such a display of sensibility!

Then notice what happens in the next chapter: in her determination to go down to dinner, Marianne gives a reason -- "the bustle about her would be less" -- that sounds more like something Elinor would urge!