I'm not sure if this is canon or fanon anymore, but I seem to recall somewhere that Veritaserum is not fool-proof either. Since I'm not sure which of the two that is, I'm going to just side with "fanon" and say that even if it is canon, it's unlikely that a 16 year old would be able to resist it...

Second, your thingum about Harry and book 7 made me think... With how closely Rowling is connected to the movies, you'd think that Lavender plays a role in book 7, too, why cast her otherwise? (Same really as with Luna... she's a side character at best, yet they went through great pains to cast her...) Hmm....