Ebay Trading API

I’d really appreciate some pointers on the ebay Trading API as I’m struggling to clearly see how it should all come together for a project I’m working on.

I’ve successfully generated xml format queries and parsed the responses. However, there are two areas where I can’t work out how to get a full, robust and reusable system in place.

First question…
On the GetCategories API call, I’ve found a couple of sites which list the top level parent categories but if I didn’t know what these are, am I just supposed to guess that, for example, categoryID 58085 is Computers and Networking before drilling down through the results? Or is there a way to do this in a tidy way that will always match the latest API.

Second question…
If I work through to the last child in the category list, there are categories that don’t have any additional sub-categories but seem to split into approved themes like genre and platform. How would I know what the legitimate options are within the last child so that my product listing would appear when someone selects something like Xbox 360 from the Games category?

I think I’m missing something but can’t find the answer in the ebay support. Any advice, suggestions or overview would be very welcome.