I think Karen is vulnerable to one thing, Harry. Her becoming part of Winter is most realistic to me in the sense she might get to be with him. Hand maiden of Molly or something like that.
Vulnerable to Harry, perhaps, but she fears Winter more than Harry does, much of the distrust and fear she has felt, perhaps mostly fear stems from that. Perhaps she'd reject the coin, but Nic knows her weaknesses... Consider how he tricked Harry into picking up Lasciel's coin bare handed without thinking to prevent little Harry from doing so. In the end Harry rejected the coin but it took a while and Lasciel did fill a lot needs he had.. So don't be so quick to think that Murphy will reject it out of hand, she is only human after all. Offer her her complete physical strength and speed plus more to help Harry in the fight? She might think if Harry was able to resist the temptations of a full partnership while benefiting, that she might also.. All I am saying is don't underestimate Nic, if there is a way to do it and screw Harry good, he will do it, Murphy would be the perfect pawn is all I am saying.. And who knows that strange dream Harry had in Skin Game of Murphy with Lasciel's coin could have just been a warning, but it might also be foreshadowing to be sprung on us in a later book.