Congratulations for such a marathonic research that lead to this clear and complete article!
We are also suffering from Google’s invalid_grant error in our Forms Service Provider that should integrate Form results with Google Spreadsheets.
We wish our provider was as persevering and strong-willed as you are!
I will send this article to them in hope your research will inspire them to keep looking for a solution, instead of just giving-up and inviting us to go somewhere else.
Thank you for sharing!!!