Don't worry about it, Andy, we all started out knowing very little. I really believe in the power of WordPress, which is why I help people on these forums.
Just out of curiosity, do you know HTML or CSS? How about PHP? Have you used other CMS like Joomla or Drupal? Just trying to get a feel for your background knowledge.
And I do agree, it would be good if you could change more things (like box sizes) without having to resort to Child Themes, and I think that's a long-term aim of the WordPress development team.
If I recall, your Child Theme won't remember things like custom background colors that you may have set up, but you can use the "Live Preview" tool when you switch themes to get things set up. It's hard to explain, but use it and you'll see what I mean.
The "default code" it uses is all the code from Twenty Twelve. If WordPress needs code, it checks your Child Theme - if it doesn't have any code for that, it falls back to Twenty Twelve.
So it's essentially a mirror of Twenty Twelve, and you can draw on top of the reflection.
I hope this answered your question, I wasn't 100% sure what you were asking.