And C'baoth killed that Dark Jedi and became the guardian himself. It is logical to asume he took at least took the Dark Jedi's saber somewhere to prevent his new subordinates from ever using it against him, or to put it in his throne room as a reminder of his prowess.
Maybe, but it's also implied that C'baoth is the only Guardian of Mount Tantiss that ever was there (reference: Dark Force Rising Sourcebook, I'm told).
The basic reasoning behind my own "Galen in Thrawn Trilogy" idea is because some people really like endings that are as ambiguous or open-ended as a game ending where the player doesn't know which
Galen survived in the end.
Not that I think any such story should be canon, however, but it would still be interesting to see unfold.