"how can I make the category description (and additional field) display on the full article page?"
This is not possible because a category and its description is only displayed in category view, not in an article view. You could probably have a custom module in a position above the article that pulls the cat desc via a custom sql query, or you do a position/variation override and pull the data from the $cck object. I dont know if the cat desc is part of that but you can check that with a an echo $cck.