Users with 10 or more reputation can answer a protected question. However, the +100 account association bonus is ignored for this check, so you must have earned 10 or more reputation on that specific site to answer a protected question.
If you use the @ to tag the name of the person who posted the question or answer it will be stripped out automatically as it is unnecessary. They will be notified anyway.
Once you start typing a name after the @ sign autocomplete depends on how long ago that user was there, I think. Certainly just typing @ shouldn't suggest a name until you start typing ...
This usually happens when posts and suggested edits are attributed to the Community user (as part of the procedure followed when deleting a user). This will eventually be fixed by a scheduled task and the reputation will go back to 1.
It looks like this is outside the scope of our site. This image is not owned by us but a third party (I think https://github.com/51cards/Stack-Exchange-Bounty-Ad).
As far as why the information is wrong, I would blame caching. There was a point when this information was true. There were two bounties worth 550. Then two more bounties were added bringing ...