Ok, I just read your discussion on this topic.

Actually I neither got the impression that Moridin made the apparently saidin-Gateway, nor that the messenger is an Asha'man. As said, maybe he had a ter'angreal or otherwise somehow the ability to create Gateways (either as a channeler or perhaps from the DO).

In any case, it seems quite careless of Graendal to just leave Moridin's servant in her lair, after all he could spy there (at least there is no mention that the messenger followed Graendal through the Gateway). Although, if Graendal isn't even afraid that the Nae'blis might poison her, she also might not care if he uses a spy in her power base. On the other hand, also other Forsaken would have the opportunity to use the Gateway to enter Graendal's lair, if it was left open the whole time (as Graendal indicates), but Graendal doesn't seem to care that much.