I downloaded youtube-dl from upstream and it works fine for me in ED.
I've already had mpv installed in ED, which in Jessie depends on libav.
libav is/was(?) an ffmpeg fork that was used in Jessie, because ffmpeg was a little sleepy at that time.
I have no idea what happens if you try to install actual ffmpeg in Jessie. At least it would collide with libav.

1. As I wrote in one of our PM's, libc6 is set on hold in Jessie for a reason. Don't install any vanilla packages of it without patching them!
I already built the latest glibc packages for Jessie and I'll upload them once I upload a new image.
Besides, there's little point in updating glibc. It doesn't change any functionality and the security point is kind of moot anyway, because even the latest packages are 2 years old by now.

2. If apt asks you to do a dist-upgrade, it means that your installed packages are inconsistent. That may very well be the case, especially if you installed my Midori 7 build, which includes a lot of backports.
Please provide more accurate information! The exact error message and the output of dpkg -l would be useful.