Seeing some of her other movies, I can see her being able to pull it off well. Also to note, you don't really see Moiraine "unbelievably pressured and unsure behind the scenes" in the first book. It is more when Rand starts doing his own thing, not revealing any of his plans, that she shows more expression outwardly than just being "cold" and emotionless. Minus private conversations with Lan. Even with that, as with most Aes Sedai, those expressions are very small. I would say she conveys her genuine care by her actions in the long run more than her outward appearance.
I always imagined Lan similar to how Viggo Mortensen portrays Aragorn in Fellowship of the Ring.