"But I found that some videos have several videoplayback strings. Haven't figured out this part."
Firefox does this much better.
"Downloadhelper" is one Firefox extension that works.
There are others that work, as well.
In fact, Firefox works -so well-, that I don't bother attempting to download videos (or audio) with Safari anymore.
I still use Safari as my main browser, but -- if I come across a video/audio clip I want to keep, I just copy the URL, open Firefox, and "grab it" there instead...
The "activity" window was removed from Safari (or, made much more difficult to use) as an attempt to purposely limit the kinds of downloading that could be done with it...