Here is my reasons for Taim maybe being Demandred in disguise. Many others have used Taims looks as a reason, but Demandred wouldve suspected that Rand, being Lews Therin reborn, could identify him from his looks. If Demandred used inverted weaves to change his looks, it could be possible to dupe Rand. But, we learned in Ebou Dar, by Elayne and Nynaeve, that if the look you weaved was too far from your own face in simmilarity, it wouldnt hold. If Demandred made slight changes in his looks (he had the bold nose and "hawky" expression from before) it would be impossible for Rand to suspect "Taim".