Lee,
I can answer the keyword questions:
Categories and Keywords are the same objects. It is just a difference in the CMS interface and documentation. A cr_keyword with children magically becomes a "Keyword Category". That is all. otherwise they are exactly the same. I am planning on a user interface to the keyword services seperate from the CMS package and I think having two different terms for a top-level keyword with children and one without is confusing.
So to answer your question, keywords may be arranged in a tree, but it is not necessary.
