] The other two are ambivalent. She's not SURE whether she can venture to laugh can be taken to mean either (1) she is not sure it is polite to laugh at the foibles of others (2) she's not quite sure if he is joking.

Well, she just doesn't know him, period. She doesn't know if he's joking or serious and, like a girl on her first date, she doesn't want to make a misstep, doesn't want to offend, etc. I think she comes out of the encounter pretty well. She is honest, but censures herself, she is able to contribute to the conversation, even if she's a bit bemused. She's doing fine, our girl. ;-)