> "Is it possible to clarify this bit of the story (...)"
Yes, it refers to all the Candidates of the current generation who didn't get elected as High Priestess. It's basically one last adventure for the gang.
From the previous generation of Candidates, the ones who are alive are Drishtya, Shrezdill, and the Gaanidan one. Leap dies during the story, the Aiishen Candidates and High Priestess die before the start of the game.
> "In my speculation, as I understand, the previous Gaanidan candidate, for some reason, goes into hiding and secretly teaches Artume, because the way the text is written, it might be suggesting that?"
This is correct. The plan was that the previous Gaanidan Candidate left her responsibilities, became an hermit, and straight-up just let herself get captured by monsters repeatedly, thus learning their various magic abilities, and short years before the start of the game, Artume finds her and becomes her disciple. The protagonist was going to find the previous Gaanidan Candidate at some point during the third adventure, but it was so disconnected from all the other plots of the story that I just forgot about it.