I am fascinated by your site - presumably down loads like the dynamic footer might be down loaded as part of the child theme?
Good news is that twenty eleven has a dynamic footer, just add widgets to footer one, two or three, and the footer will show and resize based on the number of activated widgets.
Nearly all posts on my website have a theme download, this is in most cases a functional Child Theme in its own right, and can be activated for testing, the idea being that if you like the changes you can then merge the files, functions and calls, into your own child theme, so like a code reference library, and all free :)
...So in order to adapt it, I have to input the relevant HTML (with the import rule)right?
Wrong, if you wanted to change something in the header.php you would copy the one from twenty eleven, save it in the child theme and make the changes in the child theme, if WordPress finds a file in the child this takes priority over the parent!
A good way to get started is not to code but investigate, look at a few tutorials for child theme, get your head around the changes, download any free child themes, these will only have a few files, again investigate.
And in order to ensure these (CSS) files are read, I need to upload them via FTP?
You can edit the files from Admin > Appearance > Editor, but make a mistake can cause an error and you will then not have access from wp-admin until you download and correct the file.
I'm not sure what you mean by do the development on a local install; what is a local install?
A local install is a whole WordPress environment on your local drive, this will include php, WordPress, phpMyAdmin etc:, it is just the same as online, if conneted to the internet you can update, install plugins, just like online.
Local Installs: XAMPP, LAMPP, WAMPP, InstantWP, WebMatrix.
If you look at the number of topics here that are where code has been changed on a production website via the admin editor, and the lockouts that are caused, you will see the benefit of a local test website.
Here you can easy develop and test themes, changes and plugins for your Website offline, make a mistake it is just a local file so no problem, a bad plugin just delete the folder.