Re: Problem with MIDI.
So, re #3. .In Audio MIDI Setup there's a second window showing the current configuration of MIDI devices -- if it's not showing, go to the Window menu and select Show MIDI Studio.
Re #5.. All applications maintain a Preference file, with the settings you dial up in that program's Preference panels [i.e. GB > Preferences menu]. Sometimes this file get corrupted, and if you remove it the application will automatically create a fresh one, which occasionally solves unexplained problems. However, GB seems to manage this differently - I can't find them in the usual location, and an Apple Help document sends you to the Terminal [!] to reset them, so forget about that..
I'd lean to #4, check w/ Korg on the need for a [new/updated] MIDI driver -- it may be a red herring but at least it's worth eliminating..