The fact that he can sit there and talk like that... Darcy's instinct is to get mad at him for being so calm, because it makes her feel like an idiot for freaking out, and why shouldn't he be freaking out too? Freaking out is a totally legitimate response to this shit, isn't it?

But he's not, and Darcy is smart enough to realize that it helps. It helps a lot. But it also leaves her not knowing what to say, and so she nods, and stands there feeling dumb and awkward. Is she supposed to just turn on her heel and march back off to base now to catch up with the other liaisons?

No. What she does is reach out to wrap her arms around Severus' torso, side of her face pressed against his chest, in an embrace that's more serves to anchor her than it is anything terribly intimate. Apart from the fact that an embrace is by its very nature rather intimate. "I am really scared, and I don't know what to do but stand here useless. But you seem to know exactly what to do, even when there's nothing you can do. And all I want to do is cry, except I really don't want to start crying right now." All of this uttered in one breath, pace picking up all the way through, until the last few words are all kind of one compound one.