Thanks for the quick replies. I'm still interested in the several other questions I asked, if you have any ideas on those.
Regarding deletion (question 2), as I say, I had to uninstall - so, yes, Norton 360 was installed previously.
You were on target in suggesting that there could be other folders that the uninstallation did not remove. In fact, there were. An Everything search found seven Norton folders, along with the one Symantec Shared folder in your list. There was nothing in the Downloads folder.
I decided to reinstall, but without installing the patches.
Then, having already tried the Windows 10 uninstaller in Programs and Features (via appwiz.cpl), and skeptical of Norton's own uninstaller, I tried using Revo Uninstaller (free version) to uninstall. That kept crashing. Presumably Norton 360 was designed to resist malware that might seek to uninstall it. Revo supplied the product code needed for Microsoft's Program Install and Uninstall Troubleshooter (Not Listed option), but that troubleshooter didn't help.
It appeared that Norton's uninstaller might be the only choice. That was annoying, as it did not seem to provide any way to uninstall until I had activated it - which seemed problematic, given that I was already using Norton 360 on the maximum three devices.
I didn't understand your reference to "rnr" but, with further searching, found that you probably meant Norton's Remove and Reinstall tool. RnR said it would remove and reinstall, but its Advanced option allowed me to remove without reinstalling. That insisted upon an immediate reboot.
After rebooting, I found that Norton 360 was removed from appwiz.cpl; its shortcuts were gone from the Start menu; the 4.4GB file was gone from C:\ProgramData\NortonInstaller\Settings (indeed, that folder was empty); and Revo no longer found Norton 360. The Symantec Shared folder was still there, as were six of the seven Norton folders. Most of those folders still contained files, but I was able to delete them.
CCleaner did detect some material for deletion, though none specifically naming Norton.
For this one of my four questions, each of your suggestions contributed something. Thanks again for that.