I could easily see RJ writing out the 'ride into the sunset' bit many books back with intention to flush it out or rewrite it and Sanderson not wanting to touch it even if RJ would have rewritten it to be a bit more logical. Like how the supreme commander of the shadow, a guy with obscene knowledge and power who previously brought down Hawkwing's empire pretty much on his own and has a rap sheet worse than Hitler would be left alone unguarded while still breathing. Or that on finding his missing body a hue and cry of epic proportions wouldn't be raised. Cadsuane could probably paper over things with a 'he is dead and his body was cremated, you know I can't lie' but the whole idea of Moridin being unguarded was a stretch.

Overall, while I loved the book and series, that scene was a touch clumsy and I'd rather assume BS had a working copy of it he didn't want to screw with more than the absolute minimum and that RJ wrote it casually assuming he'd iron out the wrinkles when the time came. Or If RJ's notes had said 'only the 3 know, but Cadsuane guesses' I's bet on BS feeling obliged to keep it that way, for the final scene.