1/ Yep. Not sure about the UI, though. It defaults to Serif_Bold or Serif_Regular depending on FW, but it might very well pick some stuff from the CJK/I18N fonts when encountering non-latin scripts. And, no, you can't control the styling on the CJK/I18N font, there's only one variant.
2/ No. It doesn't block updates, and it's completely transparent for the update process, even if said update wants to update default fonts/ss.
3/ Everything's backed up automatically, but nothing is actually ever written on the system partiton anyway (hence why we're invisible for official updates). Disabling the hack restores the system to a vanilla state, even while it's still installed.