I think Sidious is wrong about Moridin laying down the Compulsion. The scene in question takes place in the prologue of The Path of Daggers, so it will be easy with anyone with the book handy to give it a quick re-read (I don't have mine handy right now). But isn't it odd that Verin laid some form of Compulsion weave on Elza, and then, later in the series, SH instructs Semirhage to remove a weave of Compulsion from Emma's mind? It just fits together too nicely--SH/Moridin sensed the weave in Elza, realized it wasn't of their doing and probably stood in their way, and instructed Semi to undo it.

Most likely, they realized it based on her actions. My guess is that SH/Moridin instructed Elza to perform a task against her Compelled command and quickly noticed the typical signs of one Commanded to perform two contradictory tasks.