FWIW, my threads from past courses are saved on my mac as .webarchive files, and they *are* local to my mac. Without a wi-fi connection active, Safari still opens the threads and I can view all the text of the threads normally, with the exception that the videos don't play. Once I connect to the internet, I *can* play the videos in my saved .webarchive files, even from courses that have been taken down from the forums months ago.
To summarize, in Safari, saved .webarchive files will give you access to all parts of threads from long-gone forums, including video, when you are connected to the internet. When offline and not connected to the internet, you will still see and be able to read everything but the videos won't play.
So .webarchive files have worked very well for me to have unlimited access to old forums.
Hope this is helpful!