What???

You know the red progress bar that's under a video when you're watching it that tells you how long the video is/how much you've watched? (pic related)
It's meant to resemble one of those.

The fact that the red line is there means you've watched the video to completion. A video you hadn't watched all the way would have a partially complete line, and a video you haven't watched wouldn't have a line at all.

I'm not sure, since I don't have a Youtube account. Since it is only found on the videos you watched, it might show that you've already watched them.

As far as the progress bar, that's what was happening to me. IDK about the rest.