I don't think Marianne *feeling* like having an outburst is something to censure--indeed, given her history of saying whatever is on her mind, only feeling like having an outburst is something!

I do see this scene as demonstration of a change, as before Marianne would have quit the room, or not held her tongue. True, she is being more polite out of affection for Elinor, but surely that is a step in the right direction? It is, at least, putting someone else's needs above her own, even if that unselfishness is sprung from guilt and a need to atone.