My original posting was flagged as duplicate of a 2019 post, but the solution in that 2019 post no longer appears valid due to Google very recently eliminating "googleusercontent.com" as a valid URI or javascript host domain, as described in this recent post, which I referenced in my original question.
So I'm asking again for help--either to confirm that Google no longer permits use of its federated login for Google webapps, or clarification of how it can be used (where webapp is operated under the "Me/owner" account) contrary to my reading of "this recent post" cited above. I hope re-submitting my question (with this explanation), or asking for confirmation of another posting, is not violating Stackoverflow protocol.
Thanks again.