Absolutely no other software has been installed and no backups restored. Simply OSX, Steam & Steam Games.
What is the build date of your Steam client? It's in the "about" menu. Mine is still Oct. 22 and doesn't have any of the problems you describe. I know there's a more recent version (dated Nov. 11-12) and I'm wondering if that release hasn't caused the additional problems you're experiencing.
I tried to update my client, but the "patch" wouldn't download, so I'm wondering if perhaps Valve pulled it for the Mac because of the issues.