I think it's more likely a random Shadow channeler that got sent in to make sure Lanfear was killed. He might have been told it was a suicide mission, he might have been told he could get out alive, but I think he was told Lanfear needed to be punished, and they were sending him in to kill her, along with some warnings and advice about the Finns.

He could have destroyed the redstone doorway, that's not a bad idea.

I just don't think Moridin would risk himself in the Finn's realm, especially after we heard how dangerous it is to mention or ask questions about the Shadow.