@ttzeng Got it! Google IAM/Service accounts for the project were mis-configured.
Creating a new Google project generated pristine IAM/Service accounts and fixed it. Was not able to identify the exact issue with the previous IAM/Service accounts. Also had to ssh-add the client computer’s private key. Was able to specify a custom username as you described in the guide.
Knowing that you validated the guide helped me stick to it and know that the issue wasn’t Clear Linux. Guide worked exactly as you wrote it. Thanks for all your help!