I have 3 Add-ons installed: Provider for CalDav & CardDav, Provider for Google Calendar & TBSync.

Which one are you actually using? Provider for CalDAV & CardDAV, or Provider for Google Calendar? It's either or, you can't use both.

I disabled Provider for CalDav & CardDav. That solved the issue of a Google calendar entry showing up 2 times in Thunderbird calendar. However, new Thunderbird calendar entries still do not get copied to Google calendar.

I suggest you use TbSync and its Provider for CalDav & CardDav extension and check their setup. Ditch the other provider and remove any non-synching network calendar.

I've tried all combinations of the 3 add-ons; still unable to copy events from Thunderbird calendar to Google calendar.

When you setup TbSync, a NEW calendar (your Google calendar) is added to Tbird as a SEPARATE calendar from any other calendar you had added to Tbird. So, if you had added a local calendar, it does NOT get connected to a network calendar (Google calendar). You can try exporting the local calendar's events then import them INTO the Google calendar.

Please be patient with me. I apologise in advance for knowing nothing and being annoying. Today (10 Dec 2020), I didn't necessarily mean to, but I seem to have updated to Thunderbird 78.5.1. My email was messed up for a while. Then, when getting a new email with a calendar event invitation, it would display the "no writable calendar" notation. That seems to have stopped & I can accept new invitations. HOWEVER, all of my old calendar events are gone and I cannot sync (it is greyed out).

Can I use TBSync? Currently I have Provider for Google Calendar installed. That's what I've had installed & used without much problem for years. I would gladly have continued to use the old version of Thunderbird.

I don't know if this information is helpful or not: the old events are still there on my ipad and iphone calendars; it just isn't convenient to not have them in the Thunderbird calendar. When the assistant sends calendar invitations, she uses Outlook, and I know that the email is Google-based (a separate domain, but to configure, one logs into Google).

Again, I'm really sorry for seeming dim. My Google searching leads me to believe that TBsync doesn't work with Google calendars. Is that inaccurate?