That's the only explanation I can come up with, though Id like some nod to it one day. The only other option I can think of would be that the Queen of Winter is somehow immune, but CD certainly made that unlikely.
Is it not possible that she's 'immune' in so much that, if she's aware and on the ball, then like with the Gatekeeper's eye detecting Nemesis, it's
next to impossible to N-fect a Fae Queen once you get into the Mab/Titania realm. It makes sense the Ladies can be vulnerable as the youngest, least experienced and powerful of the trio. The Queens
might be able to be N-fected, but it's as unlikely to happen as anything (though some have, and will argue that Titania is probably vulnerable now in her emotional state, though I think it's overstated). And when you get to the Mothers, they are probably 'literally' incapable of being tainted.