[deleted] You can mount the guest additions ISO like you would any other disc image
Success, finally! After the first download of the guest additions .ISO file failed, and the default application to open it (Ark) failed the second time, I assumed it wasn't valid. But just to check for sure, I connected with my NAS, copied the .ISO file from the Downloads.Linux folder there to the Downloads.Windows folder, and mounted it in the Windows VM's DVD drive.
And I'm happy to say that it was able to successfully install those additions from the .ISO that appeared to me to be broken. It should have been opened by VirtualBox, and installed where VirtualBox could find it. I'm going to do that manually now.
I apologize if this thread has seemed adversarial at times. That was not my intention. In the current version of virtualbox.org, in the Downloads section, I could find no link to install the guest additions, as there was in the past. Thus, the only way I thought it could be installed anymore was via the host machine.
I'd STILL like to know where the VirtualBox installation folder is, so I can manually copy this .ISO there.