So, we have three branches:
1. Shea who is told that he can use the Sword because of a specific family line, yet not really told what the Sword does.
2. Par and Coll who know what the Sword does by listening, yet only Coll can use it because he doesn't realize until the end that he can.
3. Bek Rowe is told both that he is an Ohmsford, and trained to use the Sword with full knowledge of what it does.
These three stories are contradictory about the nature of the Sword. Allanon at one point is wondering why only the Shannara line, and not all elves. But we read First King of Shannara and actually see how the sword is made, the crazy alloy the sword is made with, the using of Druid's fire to forge, and the welding of the Eilt Druin crest into the sword. But there is no spell cast requiring that only the line of Shannara can use it. Bremen just decides to give it to Jerle Shannara, because maybe a king should have it.
In other words, besides the ability to break illusions and magical contradictions (such as, magic can't cause someone to live forever), the Sword is actually not about truth at all, but perception of truth. Jerle Shannara can wield it because of its name, the Sword of Shannara. Allanon assumes that only the Shannara line can hold it. Actually, anyone can hold it, they just have to be convinced they are of the Shannara line. The reason why Shea wasn't told what the Sword does was that if Allanon happened to be lying about the Ohmsfords really being Shannara family, knowing that the Sword works to expose the truth might have had him ask the question, "Am I really a Shannara?" Not only would it not work for him, but because of that, nobody in the family would be able to use it either. This is why it worked under three different circumstances. This is also why Bek Rowe is also necessarily told that he's an Ohmsford (and thus a Shannara). Maybe he's not?
But ummm, it it flies in the face of Occam's Razor. What actually happened? Brin used her Reality Warper power one last time to Easy Sex Change and marry Kimber Boh. That's crazy, right?!?
Well, no, not as crazy as having a chance meeting with someone, developing a fond friendship with her to the point where only her voice will snap them out of Illdatch's spell, then instead of staying with that person, you go back home and marry someone who you're not into after he's hung up over his sword breaking. By the way, Kimber Boh's prospects in this town were horrible. We see her getting harassed at a bar, and it's heavily implied there is not one decent man in the entire town. And she's stuck mostly in the woods caring for Cogline.So which is more likely? That Kimber Boh had a memorable experience with Brin, but neither of them acted upon it, yet somehow this story got handed down this long (my family has no stories that are three hundred years old) enough so that a descendant of Brin's goes on a cross-country trip to find a descendant of Kimber Boh's (whose line is still alive, and easy to find, given she took her husband's name)? Or that they secretly married, and kept the story of their love alive by telling a slightly true story with some key facts omitted or changed? Allanon even warns her never to use her magic again, like several times. Now, what might she use it on...?