I don't think the site is a mess. My videos do not automattically advance as the OP states. When one is done I get a choice to either replay or advance to next video. So perhaps that is a setting somewhere.

What I do agree with is that the ability to uncomplete a lesson needs to be fixed. I often jump ahead within a lesson to print out all of the notations for that chapter so I can have them all in my notebook before I start. When I do that those lessons show as being complete when all I did was print the notation. I would really like to be able to put them back to uncomplete either one by one or the entire lesson. This was pointed out a long time ago and one of the instructors (I think Christopher) chimed in and said that the compete buttons are toggles and you can go back and forth. He thought that function worked. We then challenged him to actually try it and he came back and said that we were correct, it does not work. So it seems the intention is there but not programmed correctly. That should be fixed for sure in that case.