This was a very odd one to debug. :)
But, I was glad for the challenge.
It turns out there are several things going on here, only one is related to P2.
1. Author pages for users not on the current blog should return a 404, not show you their full name. This was the only P2 bug, it's now fixed.
2. Everything else related to /authors/brent/ and /authors/brent-2/ was more of a WordPress quirk, not a P2 theme bug. In fact it was a very weird data conflict due to you (Brent) being one of the oldest WP.com users, ever. (Congrats on being an early adopter!)
You signed up way back in the day when the "nickname" and "user_nicename" were tied together. WP changed the way it handles user_nicename since then; now it's the same as your user login, a lower-cased slug, essentially. So all along your user_nicename should have been 'vanfossen' but since you'd been using 'Brent' as a nickname, your user's entry for 'user_nicename' was incorrect. At some point, probably based on a code upgrade, WP.com tried to create you a user_nicename with the value of 'brent' — but it was taken by another user already, so it made it 'brent-2' instead.
All URLs for your user on WP.com should now use 'vanfossen': the WP.com forum profile, any author URLs (with any theme), URLs in Author and Author Grid widgets that you seen in a sidebar, etc. It's kind of your permalink around WP.com, and it's working correctly now.
Let us know if you see any other issues.