Thanks Alfred, can never tire of seeing A Big March underway!
I had the same a couple of years ago Steve, with a pile of dead and dying black bees shoved out the entrance and a few hanging around the entrance looking very sick and shivering. I opted to wait and see. Likewise they recovered on their own, thankfully.
A shook swarm onto foundation for a badly ailing colony seems a bit of a kill or cure. The one in the video didn't seem to me to be particularly badly affected with CBPV, which was good to see. I have tended to use shook swarms for other people's colonies which were on very neglected and defunct combs and going nowhere fast. I am sure that a shook swarm takes off much better if you can supply it with at least some clean already newly drawn combs to get started. I know a purist view is it should have everything bang fresh, but its asking a lot.