Your post really got me to thinking Moni. I personally didn't draw a parallel between Marianne and the child originally. But in formulating a reply to you, I think I'm beginning to see one.

But first, I feel that when Marianne reproves Lucy somewhat with "usual way of heightening alarm, where there is nothing to be alarmed at in reality." , it's an instance where Marianne is seeing and judging a situation properly. She's responding to Lucy's sad accident remark.

I really think the judgment here is not on the crying child, but the ridiculous way the adults reacted. The mother's consternation was excessive; but it could not surpass the alarm of the Miss Steeles, and everything was done by all three, in so critical an emergency, which affection could suggest as likely to assuage the agonies of the little sufferer.

The poor kid is being trained to be a brat. As a parent, I've noticed, and I'm sure some of you other parents, have seen, when your little baby/toddler, gets a scratch or bonks their head (something minor that just hurts for an instant), there's a pause where they turn and look at you to see your reaction- then the crying starts. I'm sure most of us do the "shake it off"- "oh you're okay" to try to lessen their alarm (ie. give some comfort, but try not to over do it).
This is where I began to see a parallel between Marianne and Annamaria. We know that both Marianne and her mother are emotional. If Mrs. Dashwood is naturally that way, she's not inclined to check it in her daughter (ch. 1)by Mrs. Dashwood it was valued and cherished. And you are right, she is young. There is some immaturity there that is pretty common in teenagers I think. The "know it all"ness and absolutism seem very teenagery :-) to me.

But in honesty, I see the self-portrait more that Marianne is an over-reactor. So when she talks about heightening alarm, she's unknowingly describing herself. The whole family has had a trying time. But when she increases her sadness, or in any way consciously is trying to give an "appropriate show of grief" it begins to cross over into falseness if you know what I mean.