I understand how I have created confusion, but I am taking the 730 route. I am ignoring the 740 dedicated server app because as you point out, it is now obsolete for the 730 client.
It seems you can type in any name for the beta branch and a download will start. Earlier I tested 'app_update 730 -beta fakeversion validate' (literally type 'fakeversion') and a download started, so my guess is that it just defaults to the main 'public' branch if the beta branch name is not recognised.
I'm confident 'csgo_demo_viewer' will work, based on the way branches are listed
here. 30% downloaded at the time of writing, will update once I can attempt to run the server.