Duplicate code actually makes it MUCH harder to keep track of what you have changed and it creates duplicate code which can slow down your site.
I'm a little bit confused about which theme you are actually using - looks like a forever child theme? Is the parent forever theme unmodified?
I'm also worried about putting code into the wrong places and completely screwing up the entire site.
Probably WILL happen :). That's another reason for not having unnecessary code in the child theme...it's harder to see what you've changed. Also be sure you make VERY frequent back-up copies of the style.css file.