1. No, an OBi cannot be added to another Obitalk account while still on another one.
However you can do almost everything except configure for GV using the onboard web interface. Log in there and disable Obitalk Provisioning.
Also I'm reasonably certain that Obihai/Poly tech support have the technical ability to detach an OBi from an Obitalk account. But I don't know if they will do that. Try asking them.
2. Usernames of obitalk accounts are email addresses. If you make a list of all the email accounts you have ever created
and can still access, try each of them with the "Forgot password" option at the
obitalk.com login.