OK - So I finally got all their workstations running.
After the problems with the initial install workstation (Windows 11, Version 24H2, Build 26100.3194) noted above, i.e the Adagio Installer error message;
"The file 'c:\windows\system32\MSVCR110.DLL' could not be opened. Please check that your disk is not full and that you have access to the destination directory [OK]"

MSVCR110.DLL did exist in c:\windows]system32.

As noted, we gave up on this workstation and I did the Adagio upgrade from a different Windows 11 workstation with no issues.
I then did the ALLWKST workstation install on 5 other workstations (Windows 10 and Windows 11), also with no issues.

The original workstation that I tried to install from that got the error still displayed the same 'MSVCR110.DLL could not be opened' error when trying to run the ALLWKST workstation install.
I showed the client's outside IT company how to run ALLWKST and let them try to resolve the issue and get Adagio installed successfully on that workstation.
I got it second hand, but they 'updated/copied' MSVCR110.DLL from another Windows 11 workstation.
Then, when they ran ALLWKST again, it gave a similar error message, but this time the error was about a different msvcr/msvcp DLL in c:\windows\system32.
They updated this DLL and ran ALLWKST again, and once again, it complained about another DLL
After about 5 iterations of doing this, the ALLWKST workstation install completed successfully.

I know this is not the best explanation, I have notes scratched everywhere after several days, but I wanted to let tech support and other dealers know of the solution (updating the DLL's in c:\windows\system32) in case they run into this error message.
It seems that perhaps the Adagio install needs to be updated to work with the latest versions of Windows 11-24H2 without getting these errors and having to copy DLL's from other machines.
It did not seem to be a rights issue on the original workstation.