your 5(a), 5(b), and 5© usages (the valley-girl meaning)....by the way, thanks for this post.....great summary of the evolution of the word......
I've always thought the usage of "like" in the valley-girl style was due to the speakers' uncertainty/ignorance, fear of getting something wrong, or not wanting to appear too forward or, even, smart ...thus a way to hedge their statement (somewhat close to your 4(a), 4(b), and 4© meanings, but in a more ignorant way)