Oh dear! Yeah, I think writing yourself into the situation where Mordred definitely has to cause the downfall of Camelot but also you can't bring yourself to write Mordred as actually betraying Arthur (and/or as being anything but morally justified in doing so) inherently weakens the depiction of, well, Mordred causing the downfall of Camelot. Though perhaps it's partly just Stewart rushing through to get to the end she doesn't want to get to, from the sound of it... (I really need to give this trilogy another try one of these days! As I think I've said, in my youth I read The Crystal Cave, read most of the second book, and then went back to reread The Crystal Cave a bunch rather than go on to the tragic bits, lol.)