This Nemesis character is getting credit for everything, and in my opinion it's not the end all be all of antagonists. I think there's a catch for Nemesis to infect an immortal. I think Nemesis has to strike a bargain for the infection to take:
Lea: In Proven Guilty she points out that she got too arrogant with her power and thought she could beat the "he who stalks us all." In other words, she welcomed Nemesis to try and take her as part of accepting the Morgana Athame. She thought she could defeat him...but learned differently.
Maeve: Maeve was toturous, sadisdic and viscious towards mortals (mostly) because she was envious of their free will. She, so much, wanted what they had, and took every oppotunity to exercise her anger in that regard. Then, she bargained with Nemesis for free will, and planned on using it to lie to Harry so that he killed Mab, and she took the throne. Even at the very end, she accuses Mab of being envious of her free will
Aurora: I believe Aurora initially resisted Nemesis' bargain. However, allies of the Outsiders discovered a 'mortal' child named Allison Ann Summers (born just before Grave Peril). Bianca and others caused the thread between Faerie and Reality to thin, allowing a misled Agatha Hagglethorn to specifically make claim on the infant. The infant turned out to be Aurora's. You will recall that she has a remarkable amount of mortal/changling acquaintances (brought to light in Summer Knight). Harry was able to save Allison Ann Summers in Grave Peril, but it was a close thing. Nemesis used this to convince Aurora to accept the infection, and her ultimate goal was for Winter to hold dominion such that she would be able to walk off into the snowy sunset with her child.
Cat-Sith: Not knowing this character too well, it is difficult to understand what would motivate him to disavow his Queen. However, he is the leader of an entire species. It must be demeaning to take orders from another regent. Especially, it must be demeaning to be ordered to take commands from Harry Dresden. And, considering the first time we see him after the infection, he tries to kill Harry. And, considering that he stated how much he wanted to make coasters from Harry's spine in the beginning. I think Cat-Sith was promised the opportunity to kill Harry Dresden (an instinct) if he accepted Nemesis. Even so, Cat-Sith resists to some extent.