@morriswalters
I don’t know if that’s necessarily true regarding Summerfire being the only way Harry could take down Eldest Fetch, given how well he performed in the Faerie war in SK, & he was fresh off beating a bunch of necromancers.. I do believe that Mab was not expecting Harry to have access to Summerfire, or to use it differently if he did - this is one I believe Lily got past Maeve & Mab both (mostly since they weren’t expecting her to show any initiative of her own).
Since you’re in the camp that believes that Mab allowed the assault to happen & was actually in full control the whole time (except for the Nfected Maeve part), was Harry in any real danger against Eldest Fetch? Mab could presumably have dispatched it at any time since she had run the table at this time.
Instead my alternative theory to you is:
Maeve has convinced Mab that like Aurora, Lily too has been taken (which is possible given how long Aurora had Lily ‘kidnapped’ in SK & Maeve’s subsequent manipulations). This is why Mab places all her troops at the Summer border - she is expecting the next Nemesis attack to come from there, which is why she doesn’t attack the Ramps encroaching on her territory. Her actions unfortunately mandate that Summer must respond in kind, leading to the opening of PG.
Of course the attack comes from Winter, which catches Mab by surprise but she is able to power through it & defeat the assault on Arctis Tor. Once she had won, she figured out that Molly is also part of the BC’s play. Therefore she kidnaps Molly to get her out of the Black Council’s clutches since they have been clearly planning something involving her for a while. She has Molly brought to her stronghold which she has just won in a brutal fight (hence it is the safest place in Winter) - knowing that the Black council will send someone to rescue Molly, Mab puts Eldest Fetch in charge of guarding Molly - it will kill her if Molly is found to have been Nfected already.
Maeve tries to outplay Mab by sending Harry to get Molly back (either Harry dies trying to save Molly, or he saves her & removes her from Mab’s control, perhaps saving Lea on the way as a bonus). Like the caterpillar that Lea has guarding Harry’s apt from the NN attacks Harry in Changes, so too does Eldest Fetch - it simply doing what it was asked by Mab - guard the girl & kill anyone sent to ‘rescue’ her. Mab knows that Michael is fighting Outsiders in OR so he isn’t likely to come soon. Maeve was probably fine with Harry being given the Summerfire since it will further the fiction to Mab that the Nfected Lily has Harry under her thumb. She also is the one who speeds up time Tewary in the final seconds because she wants the returning Winter armies to kill Harry - another reason why she is initially ok with Harry being given the butterfly.
Mab is relieved that Harry was sent since she knows that he is not Nfected, so allows the fight to take place - if Harry can’t dispatch Eldest Fetch, perhaps he is not yet ready to become the WK (Mab would step in presumably before Fetch kills Harry) - Harry will have much tougher opponents for what Mab has in mind. The Summerfire was unexpected, but it is what allowed Harry to win - like Odin says in CD, Harry has friends & needs them in order to survive what he is up against - Lily who likes Harry a lot personally & feels bad about manipulating him gives him the butterfly to accomplish 2 things - attack the Wellspring &, save himself. Maeve is peeved when she realizes what Lily has done, which allows Harry & his cohort to be saved before the arriving Winter armies can destroy him.