If Mab knew that Ethnui would start a fight at the peace talks then it could be for Mab? Winter must have expected the Fomors to betray them in some form, so having Mab secure by her only being present via dobbleganger would be one way to reduce the risk significantly.
Except do you really think Ethniu would have been fooled? And why would Molly need to create the doppleganger ring for Mab (Mab could easily do it herself)? Not to mention, after that assault wouldn't the doppelganger have been destroyed. If Mab had been "destroyed" I would be more inclined to believe this theory. But as it stands, I don't think this is the case.
Also, I don't think anyone even thought Ethniu was coming to the Peace Talks. They were expecting the Fomor lead by King Corb. The whole point is that those who were thought forgotten, defeated, banished etc are with the Fomor - and Ethniu had long been forgotten.
*Sidenote: Ethniu was such a bizarre choice as far as the current novel goes. She is hardly significant in terms of the pantheon of the Fomor. However, I do believe that there are reasons Jim chose her beyond "surprise minor character is actually big bad".
Back to the topic. Rashid seems like a good candidate...but I don't think he would need Molly either.
Whoever Molly created the
similar doppleganger item for, they would need to be not as magically powerful AND skillful as Molly, they would have a sudden need for the item, and they would need to have some fairly okay relationship with Winter (if not Molly) and they would also have had to have a reason to pick Winter and Molly specifically (there are plenty of other beings that could probably do this magic).
SO - Who does that leave? Thomas would have been good (thanks
Avernite) but I doubt Demonreach would have been fooled. Austri or Etri perhaps? They were/are on good terms with Molly and Winter. But considering Svartalves propensity for creating incredible magical artifacts...it also doesn't add up.
Justine perhaps - if Thomas was trying to keep her safe. But where would he get the idea? Does he even know such things can be done?
Marcone has the same issue - knowledge. Not to mention, why do it? And why not use Monoc Securities?
I doubt any of the Knights of the Cross would bother either.
So my only guess left is that maybe Molly did it for her own troops. Perhaps to misdirect enemies and convince observers that there is a large force somewhere it isn't, while the real force gets in position.