Ivy snubs Dresden, an Emissary was to be appointed from a neutral source in relation to Thomas case, she is on the list of Emissaries in a previous case. The Swartalves and the White Court would likely have approached her for that role in this. No mystery, just a book cut in half with her role as Emmissary in Battlefield. I suspect Ivy is one of the few powers who does not formally collect favours, ruling them out as neutral.