Another solution to this problem would be competition. If there were a workable alternative to Gmail that had account recovery mechanisms better suited for this population than the librarian could simply recommend that for their patrons. Can anyone suggest what a good alternative to Gmail would be for this population?
I would hope that libraries would have fixed IP addresses, so surely that could be factored into the authentication process? Some FIDO keys used to be able to be bought for as little as $5 (https://wiert.me/category/power-user/security/u2f-fido-secur...) so it shouldn't be beyond the budget of any self-respecting town/county/state?