I think it's more complicated than that. There's a contradiction: it has to be a group tough enough that they can do that kind of damage in the heart of Winter. At the same time, they have to be weak enough that Mab didn't feel like there was any real danger for her interests (otherwise call in reinforcements). WoJ says that Wild Hunt + Erlking + Namshiel would be iffy about Mab even needing to make an effort to stop them.
Altogether, it makes it seem like Mab made a judgement call and showed restraint towards the attackers for an unknown reason. Harry didn't notice any enemy bodies Mab left behind, but her guards were all killed. In the end, the attack doesn't seem to have met its objective either. Mab still has Lea on ice (if that was even the goal at all).
My theory has always been that Molly was the focus of PG because someone knew (or believed in the possibility) that she would one day become the Winter Lady. The fetches were intended to draw Harry's attention to Molly's use of black magic, because without him, she would have certainly been caught, tried, and executed. The main whole-series outcome of Proven Guilty is Molly becoming Harry's apprentice, setting her on a path to replace Maeve as Winter Lady.
Personally, I find it convenient that the first fetch assault accomplished three things: it sealed up Pell's theater, attracted Harry's attention, and framed Molly's... boyfriend? Side piece? I don't know what the cool kids call it these days. Molly was a beacon, and so was the guy (because he was the target of black magic and had the scent of fear that attracted them).
In my opinion, the Arctis Tor attack succeeded at the intended objective: clearing the way for Harry's party. Harry, Thomas, Murphy, and Charity barely had enough in them to handle the fetches; if Mab's elite bodyguards were still kicking, they almost certainly would've failed.
The players we know about:
1. Mab
2. Maeve
3. The Arctis Tor Attackers (speculated in the books to be Thorned Namshiel; at the very least, it appears to be someone capable of using Hellfire)
4. Molly
5. Lily and Fix
6. Nemesis
7. Rashid
Other potential players:
1. Uriel (or other agents of TWG)
2. A third party who may or may not be misleading the Fetches
I believe that Rashid knew Molly would be important, which is why he got Harry looking in on it early. I don't know if Rashid knew that Molly would be the Winter Lady, but he seems to know a lot about what's happening and what's GOING to happen. I think he at least knew that she would be a player in the future, and that her death at that point would be catastrophic. The results of her death would be:
1. First, Harry would go berserk and die. At the very least, he'd be removed from the Council. That's where he is at the end of Battle Ground anyway, but now he's got other Powers backing him up. In Proven Guilty, he had nothing but Hellfire at his disposal, along with being pals with a pack of werewolves and Murphy. Now he has: Winter Knight Mantle, Mab, Molly, the Alphas, the Knights of the Bean (maybe, I haven't decided how important that compact is yet), Bonea, Demonreach, Soulfire, Lara, Odin, a castle, and the 'Za Lord's Guard, plus a reputation.
2. Maeve's gambit in Cold Days might've succeeded. Demonreach might've been breached. Big death bomb.
3. Maggie would've been sacrificed, Eb would've died, and the Red Court would still be alive.
4. White Night might've worked, too. Elaine would've died (also speculated to be another Starborn) for certain.
5. Failing all of that, Ghost Story would've worked as Corpsetaker planned, and a Darkhallow might've been performed for real.
6. Michael would probably have remained a Knight for a while longer if he even still lived.
7. Peabody would've gone undetected.
8. Most importantly, even if Harry lived long enough and Cold Days still played out the way it did on page, there would be another Winter Lady right now. If it was someone of Mab's choosing, she'd likely be an awful lot colder and more ruthless. I think Molly's role as Winter Lady is to be a more humanizing element to Winter, which I think is going to be critical in later installments.
Etc., etc. Basically, if Molly bites the dust and Harry throws down with the Council, everything goes down with them. So it's my belief that someone (likely someone
s) knew she was going to be important, and helped push the attack on Arctis Tor to make sure things worked out as they did.