Two quick questions
How did the police know about Margot's trip to Buenos Aires?
Why does Helene seems upset when hearing the name "Caroline Parreire"? (this was the identity under which Margot booked her flight)
1 You're forgetting there were 2 seats booked; one in Alex's name (his name would surely be flagged by authorities) with "Caroline Parreire" booked beside him.
2 It wasn't explained explicitly that I recall, but the name "Caroline Parreire" clearly resonated with Helene and I think initially led her to believing that Margot was still alive.