Google Calendar fails authentication

I'll put this here for want of finding a better place in the hopes it might help someone else avoid the hours I spent searching for answers online. I am using the Yosemite Preview and have three Google accounts but only one would not sync the Google Calendar with the Calendar app when I switched from Mavericks. No helpful error messages anywhere and the Calendar app would work the CPU until nothing else would run except the fan.

Eventually, when trying to add the calendar back as a CalDav and not a Google Calendar, I came to a panel which said Google required verification (first hint!) and the in-panel browser did automatically log in to Google but showed the basic Google+ account page and did not show a page where such verification could happen. A little more searching based on the language in the error message did lead me to find this:

https://support.google.com/accounts/troubleshooter/3178296?hl=en

and that led me to

https://www.google.com/accounts/displayunlockcaptcha

Once I visited that page and logged in there was a simple response that I would now be able to allow additional devices to access my account.

After going through this process and then checking Calendars in the list of available services for the already-created Google account in the System Preferences the Google calendar appeared in the Calendars app and there was no more CPU overload.