"He's just trying to make us pity on him. What an imagination he has!" (Elie pg. 17)
I think this mean that he has made the whole story up just so they will feel bad for him. He is the type of person that would use his wild imagination to make the story up just so they would take pity on him. It means that he wants everyone to feel bad for him and help him.
"However, our first impressions of the Germans were most reassuring." (Elie pg. 19)
I think this means their first impressions of the Germans were right. They were supporting the Germans and in return the Germans were treating and being polite to the the Jews. Thus, giving the Jews a reassuring feeling. They never would ask for anything more than the impossible and they never put them down.