I am operating on the basis that a power as dark as whatever is behind the athame, that appears most likely Outsidery
I don't follow the train of logic that leads to this conclusion. The athame certainly has dark associations (according to the WoJ that nailed down it's historical reference), but I don't recall any evidence for an Outsider linkage.
Now, I certainly agree with the next step in your logic.
If the athame has an Outsider power-linkage,
then significant negative effects on the wielder are highly likely. However, I don't see why you wouldn't call those effects "corruption," especially if you are describing Lea's mental state while she was frozen in Arctis Tor. Something had clearly messed with the inside of her head. Given her reactions, "twisted by dark magic" sounds like a perfectly valid explanation.
Lea acquired the athame and (presumably) used it offscreen at some point. Some time later, Lea appeared to be suffering from the corrupting effects of some dark source of power, maybe Outsider-linked. There are two relatively obvious potential sources of this damage/corruption: the athame itself, or whatever Lea was doing with the athame. Since the athame isn't a vector of corruption (WoJ), I'd like to know more about what exactly Lea was doing with the athame. "Trying to overthrow Mab" is one idea, but that theory depends on the athame acting as a corrupting influence, so I don't buy it. "Fighting--and losing to--something powerful and Outsidery" better fits the data, in my opinion.