That is even more unlikely than the idea that Moridin might pose as Taim (which is also clearly wrong, IMO).

Taim is the most time at the BT, while Aran'gar was sent by SH to the Rebel AS, where she stays the most time. Aran'gar simply can't be at two places at the same time. Also Aran'gar's PoV show that she is concerned with the Rebels above anything else. Furthermore, RJ's various comments on Taim make pretty clear that he is an individual character, not a Forsaken in disguise. Taim's special knowledge would be explained, if he was trained by Ishy some time ago. Besides, IIRC Taim is stronger in the OP than Aran'gar. Finally, with "playing at her own game" Aran'gar obviously implies that another Forsaken posed as Sammael. This means Aran'gar's game was as well to imitate another Forsaken, not Taim. It makes some sense that Aran'gar was actually the person who sent the DF Asha'man after Rand in WH in a Demandred-disguise.