Look at this picture to understand :
When I want to modify an account, I can't, since it is already linked to a Google Account.
So I can create a new account. Whatever email I put in, it tries to connect to Google API.(it doesn't care about the email I just wrote, as long as it is a valid email string)
Then it connects to Google API, and invite me to authorize Rainlendar to access Google Agenda.
But if you look carefully, you notice that it doesn't ask to access my Google Account with the email I just wrote, but with an account the API knows already (I don't know how, I was not connected with any browser to that account).
Then this is the time (if I'm extra careful) when I can change account (disconnect / manage accounts / add new) and link the email address to a new account.
But once it's done, I can't ever change it.
For the empty summary / category / anything unless time, well, it's working when I create NEW event to the v3 Google Account.
So i'm working iwth both account (obsolete and v3) but add new event only in v3.
I guss in some months everything should be ok.
I really don't like reinstalling licence, it's always like I have a limited number of reinstall...