I am confused. The dress in the video is very different from the one I normally associate with Alexandra dressing up as Mary Queen of Scots. The neckline, pattern, etc are not the same. See the link below to see the dress she wore in 1871 as the Queen of Scots.

https://mqs.glasgow.ac.uk/index.php/2020/12/01/a-little-second-hand-lustre-dressing-as-mary-in-the-victorian-age/