1. Art Spiegelman depicts his characters as animals. How do you think this affects the story (and how would readers interpret the story differently if the characters were humans?)
I think this symbolism works well in the story, and if he had chosen to make the characters human, it wouldn't have the same effect. We know the Nazis were bad, but by making them cats and the Jews mice, it sort of reminds us that they are the enemy. Also, it helps the reader to identify who is who in the story. If the characters were all human, we wouldn't be reminded that the Nazis were constantly going after the Jews like cats and mice, and it also might be more difficult to identify the different characters.
2. Why is Vladek so concerned with money (ex: he insisted on fixing the roof himself instead of hiring someone) How does this derive from his experience in the Holocaust?
In my opinion, it derives from his experience from the Holocaust because Anja's family was very wealthy (so Vladek was rich as well) and even with all their money, the family was affected by the Holocaust. Their situation would have been even worse without all the money; Vladek probably realized the importance of money that way, and I think that's why he tries to save money. It may also be because he doesn't work anymore, and he needs to save money.
3. How would the story be different if we were reading what Art wrote about his fathers experience, instead of seeing him tell Art the story?
It would probably be more difficult to tell what kind of person Vladek was, and what his thoughts were, since he barely talks about his opinions during the Holocaust, and we only see him expressing them to Art. Also, it would be less interesting because the way the novel is written (or drawn) let's us see more aspects of Vladek's life, and also some things about Art. If we were just reading a book about what Art wrote, we wouldn't know things like Vladek's relationships with Mala and Art, or how he behaves now (saves money, hoarder, etc.)
Another question I have, that I don't know how to answer is: why do you think Vladek married Mala?