If Dess is the knight, she is not alive
I think that its one of the more daunting parts of being a DessKnight believer, knowing that to have her be the knight, something tragic had to have happened; she's there, people know about her, but she in her current form is unreachable, not quite the same
Carol probably knows some of what happened to her, why she is like this, and yet still has to pretend her daughter is missing
Whether you believe in Roaring Amalgam Theory or not (which if you don't know about you should def check me out!), I think the hardest part of learning about the knight is knowing what it would have to take away from the characters
Speculating that the knight is one of the people we know about does feel better, maybe the darkness just changes them differently to other lightners. But then you have to get into why that is, and the answer from there is much less satisfying
I think that the clearest answer to alot of these knight questions can be answered by Roaring Amalgam Theory, but even past that, can be answered by Dess no longer being with us