I was out of the office last week, but am looking at this now.
I would say that you would be best served by checking the event logs. The logs that you have displayed here are from the main application and may not show as much debug information as the events logs do (when the debug toggle is enabled in the specific IdP and/or lambda).
Regarding your other questions, I was able to make an advanced registration form with both an email and username input. Since Google does not return a username, FusionAuth will send you to a complete registration page and ask for a username. The unique username policy set on the tenant will ensure no username collisions.
Then my_user_name becomes appended by the unique user name constraint set on the tenant
The user can merge accounts (including all application-specific data) by requesting a merge. The requested-of account would receive an e-mail notification asking permission to merge, which would be valid for a short/customizable period of time.
We don't have this functionality exactly but have an approximation. On this user, notice the linked accounts tab. You can remove externally linked accounts here as an admin. Additionally, if you set create a pending link, as your linking strategy (instead of link on email), you can have your user login with google and associate that google UUID to an existing FusionAuth user (or create a new user).
I am not sure if this fully addresses what you are asking. My encouragement would be to review the documentation that we just released around linking IdP accounts, linking strategies, etc.
Here for any further clarification needed 🙂