Does this mean somebody has taken the photos and transferred them to tape, and that is what we're seeing here?

More accurately: Photograph -> Videotape -> Edited Videotape -> Laserdisc

If this _was_ done for archival purposes, what is the likelihood of the original negatives still surviving?

I'm not sure. I hope they do! 35mm resolves to much greater than HD even on cheap/common film stock.

At which stage was the text box added in?

Definitely once the footage was on videotape and before being stored on a second "edited" videotape since the box is partially distorted in the image I linked to above.