Hi Paul
I had something similar and was chasing my tail. Mine turned out to be a simple problem with cron and/or the CRM scheduler.
Calendar sync ran when I authorised and then didn’t run again. After a lot of code chasing and other activity, I went into the CLI and ran the cron job from there and it worked again, but only that once.
We are on a shared platform but long story short, I redid the cron and re-timed the google calendar sync function in the scheduler and saved it and it started working all the time. I don’t have a prognosis but this did do the trick 
We are on v8.5 rather thanthe more stable V7 and I am sure you have probably checked the above but just worth pointing out, as I know how frustrating these things are.
I hope this helps.
I watched your Bounceback video about 4 times to try and solve our long term issue with it. We just get IMAP handler errors - IMAP non-valid resource stream and IMAP search errors, from the bounceback function every hour between 2am and 6am, when it is set to run and no bounceback functionality has ever worked. I have given up on it, as there is a lot of stuff in the forum but nothing that has fixed it.
Good luck