"I can't kill you" can mean "I can't kill you, but I can order you killed". Saying "I can't kill you and can't order anyone to kill you" can mean "I can post bounties for your head". Saying "I can't kill you and can't order or entice others to kill you" can mean "I can lead your enemies to your location, or withhold help when needed in order for you to die". Etc. Dealing with Mab, at least in my mind, runs into the core issue of "Mab is fundamentally incapable of ever trusting anyone". There are other issues, like "she's a sadist who derives what little pleasures she still has from torments of others", but they are secondary to the core one. Because Mab can't trust anyone, she always expects and plans for betrayal. This means that any deal Mab agrees to (unless she has no other choice at all) will be structured in such a way that the power disparity between Mab and the other party either remains the same, or increases in Mab's favor. Anything else would make her more open to being betrayed than she was prior to the deal and, thus, would be unacceptable because it endangers her core mission (Mab is close to a paperclip optimizer in this regards, I feel). And since Mab, while necessary, is still an insane sadistic monster, even if one directed towards something needed, increasing the power disparity between us and Mab in Mab's favor is not something I would abide. Even if, in absolute terms, both parties might become stronger from the deal.
Essentially, all Mab's coins are poisoned.
If I recall correctly, if Neverborn let go of their hate and achieved perfect zen, they might be able to slip into Oblivion and escape their torment.