The deployment for XD had initially failed on the webcast machine with the 1603 error. I ran the test again on a Win 7, 8, and 4 Windows 10 versions and all MSI installs received the 1603 error, even on machines that were fresh and sparkly new from the VM-maker machine.
Using the setup.exe, I was able to get the error code 6 on three of the six deployments (the other three succeeded). I did change the Package "Run As" option to Deploy User (interactive) rather than the standard Deploy User. On the three machines that had that exit code, I deleted the Program Files/Adobe directory and tried again and got the same result.
I then ensured the machines were updated and rebooted those machines. Upon deploying, I had the same result. All three machines still failed with the error code 6.
So, I tried running the install from the command line using the setup.exe and a network share to the installer. Basically, this reproduces the setup exactly as Deploy would perform it but from an elevated command prompt within the target OS. This also failed with the same error code 6.
As a last test, I copied over the installation directory in full over to the failed machines. I then ran the
setup.exe --silent from a command prompt locally on one machine, ran the msi command on another, and then manually ran the msi file (double-click method) on the last machine.
On the one where I ran the setup with the --silent parameter, it failed. On the second where I ran the msi command, it failed with a 1603 error. On the third machine where I ran the msi installer manually, it also failed with the 1603 error.
The failures occurred on Windows 7, 8.1, and Windows 10 1607. All other Windows 10 versions past 1607 succeeded using the setup.exe. That is curious! I did some checking on why this might be, and indeed, XD will not install on Windows versions older than 1703. https://helpx.adobe.com/xd/system-requirements.html