BuddyPress.org

@do77 , David’s method will save a lot when you will upgrade.

Now for your questions

1.No, You do not need to copy any file(because you want the same layout, so child theme inherits it from parent).

2. Create style.css, and put there the content as David has pointed(do not miss the parent tag and theme name in comments, wordpress uses this to recognize the theme)

3.yes, then you can use firebug to override it as you have already given an example

4.You may not need to do that, because you are not using any other files than css.

5, yes, but please read about wordpress child theme, that will help you in long run.