Resolved: Description of resolution below although there are no insights to offer as the installer magically showed up in the ~/Applications folder.
AppleCare contacted me today over the phone and we had a session where they could see my screen. I demonstrated the download experience from the App Store and I was able to reproduce the issue the first time, much like the first screenshot at the top of this thread — a greyed out download in the appstore and a 450KB file.
I went about reproducing the issue a second time so that it was clear to the advisor. Unexpectedly and somewhat by surprise the file showed up in the ~/Applications folder the second time around. There was no progress bar that corresponded to an actual download, only a short progress bar with the word, calculating (maybe doing some checksum).
Although AppleCare and I never reached a conclusion on what the cause of the issue was I can assure readers here that the App Store never showed a download progress bar for the Install macOS High Sierra.app download, weighing in at 5.23GB. As the download suddenly appeared after my second attempt to download I suspect the file had an invisible flag or that it downloaded in the background while I walked away from the computer, I am not sure. The only thing AppleCare did ask me to do was to open System Preferences and click on App Store but this was before I attempted to reproduce the issue for AppleCare.
Thank you in advance for anyone who contemplated a response.
Cheers!
Alex
