>> 2. Fair use almost definitely does not apply to most youtube content : if you use content other people made (video games or music immediately come to mind) you are infringing copyright holder rights.

Have you read the YouTube terms for original content uploaded by individuals? I'm not so sure doing remixes isn't allowed. If you don't want people doing that to your stuff, keep it off YouTube. I could be wrong, it's been many years since I read their ToS.