Success!
I learned a lot. I was thinking in a Windows way to assume that on a Mac, I could format my USB drive to boot and then add the install files and be just fine and therefore my original question.
I have spent about 20 hours on this and BEFORE I posted here, I tried all of your suggestions (because I ran across them in various posts). But I thought maybe I missed something, so I did it all again today, actually trying to download more than the "stub" installer file no less than 5 times, clearing the software catalog at the command line, along with deleting the install files and emptying the trash each time - to no avail. You can imagine the irritation (the dogs hid in bedroom at one point)!
And then, I actually read this entire post and the solution for me came in the last answer:
https://apple.stackexchange.com/questions/299731/macos-high-sierra-media-installer . :
"It is indeed SharedSupport folder missing that causes this. In order to fix it, you should launch the small 'stub' installer, it will download all needed files to your home (/Users/username). You will need to cancel the installation after downloading ends of course.
Then if you just move those files to 'stub' installer folder, like this for example: sudo mv /Users/username/macOS\ Install\ Data Applications/Install\ macOS\ High\ Sierra.app/Contents/SharedSupport and run createinstallmedia again, it will do what we want it to do."
Please note that after the download ends, you only have 24 seconds to cancel it!
In my case, the mac OS Install folder (with the "all needed files") was here as "Macintosh HD > macOS Install Data" and I had no SharedSupport directory (as a sub-dir of Contents in my High Sierra installer package,) so I had to create it and manually copy the files there.
Then I ran this at the command line: sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/sierra(sierra is the name of my USB drive). And it worked!!
Thanks again for your participation here and willingness to respond.