1. Yes. Backup the config, reinstall, and restore the config. When reinstalling, use the gmirror option in the installer to make the array.
2. There is a widget for gmirror status, and Diag > GEOM mirrors for status and disk management. If you have SMTP notifications enabled, it notifies on any status change of the mirror.
The diag page also offers a way to forget disks, add disks, rebuild arrays, and so on. It doesn't let you create new mirrors, however.