I don't think Shadowspawn can enter tel'aran'rhiod for the same reason they cannot pass through a Gateway; only naturally occurring life-forms leave a mark there, and even then, only the wolves can exist there as spirit constructs. We've seen animals exist there in dreams and nightmares, but they don't leave tracks. Shadowspawn aren't encountered in droves if you try to Dreamwalk to the Blight, despite their being an immense of them sleeping at any time there, because all that is sense is the presence of the Shadow.

Also, the central issue I am talking about is not if Myrdraal would listen to Moghediens' commands - it is the fact that she doesn't seem to keep any in her forces to be easily reached. She actually seems to prefer using darkfriends in her schemes rather than Shadowspawn. I don't see Moghedien Travelling or Skimming to the Blight when she doesn't know if Ishamael/Moridin, Demandred, Graendal or any of the other Forsaken who would destroy her for this weakness out of hand would be there. It is too large of a chance to take in front of competitors. The only reason Aran'Gar/Balthamel didn't eliminate her was because he was under direct orders from the Dark One, and I believe that Moghedien realized that if she had to rely on the Dark Ones' mercy at that point, she was out of luck.

In addition, though Myrdraal are intelligent, we are dealing with the fact that it took Nynaeve and Elayne a lot of time to figure out exactly which movements removed the collar. This isn't something that can be done in a few minutes by a neophyte.

And the Myrdraal possess some measure of intelligence, but they are certainly much closer to being an animal than a human being. I.E. They know how to kill, what causes harm and take pleasure in pain but we have never seen them execute complicate strategies, play Daes'Das'Mar or have intellectual conversations with the Forsaken. It is possible a Fade would accidentally tear Moghedien's throat out during his attempt to remove the collar.