Folks, wanted to follow-up on this thread. For the last 3 updates, I was only able to successfully install the update so long as I loaded a command prompt as Admin, and then executing the .msi from the shell. While that successfully installed the updates, I'm unable to drag-and-drop books into the interface since it technically runs as "Admin" and not as my logged-in user. This was, as you can imagine, extremely annoying.
I did notice in subsequent messages in this thread that there may be issues with non-default installation paths. My C: drive is an SSD with limited capacity, so for years I had Calibre installed on my D: drive, with most of the software, as it's a higher capacity HDD.
I decided to symbolically link the "D:\Program Files\Calibre2\" directory to "C:\Program Files\Calibre2\" as symbolic links have been supported in Windows since Windows XP (I use Link Shell Extension here to provide a GUI interface for the functionality:
https://schinagl.priv.at/nt/hardlink...extension.html). So now, I have a symbolic link on the C: drive (default installation path) which actually resides (and takes up capacity) on the D: drive.
Ran the latest update via regular install and it worked without a hitch! So, I suspect there's something going on in the last few installations that seem to be hard-coded for the default path and doesn't respect the actual non-default installation path.