RJ needed to stick to his guns. If you set up Taim as Demandred, you need to follow through, not back down cuz it was obvious.
Even outside of Taimandred and all that unfortunate nonsense, Demandred has huge problems. His intro in LoC was okay, but at Shadar Logoth . . . yes, he was facing a circle of three, but thats a terrible narrative aspect, you can't just have a Forsaken, one of the greatest men of the Age of Legends, beaten in his intro. Give him something threatening to do, like killing an Asha'man or two before being scared away. Just running away was poorly done.
I wish parallels between Lan and Demandred had been set up, even if it had something to do with their backstories. Have them meet at some point, or have Demandred be the heir to a destroyed kingdom, or have Lan be second to some sort of Tower Warder. Those two don't work well together with no connections. Establish SOMETHING, even if it's minor. Maybe even have Rand draw a parallel between the two at some point.