It would not have been a treacherous gift if she was already infected. It all makes it more likely that Cowl himself was infected.
If we had any proof that the athame could infect anyone other than Faerie, this would be a more appealing theory to me.
Harry jumping on Nemesis as an explanation for all sorts of weirdness in CD doesn't change there being plenty of other perfectly satisfactory explanations for all of that weirdness excepting only Aurora going loopy prior to CD, and is notably similar to Harry leaping on the "Black Council" as a one-size-fits-all explanation at the end of PG, which he's been notably quiet about since the Nemesis concept came along.
(Since the whole series is largely a detective story, it's an article of faith with me that
any "one theory explains most of everything" that Harry comes up with before the end of the last book
has to be wrong. There is no "Black Council", just a bunch of different enemies with different agendas all sometimes working together and sometimes screwing each other over, and Nemesis is most likely, and most parsimoniously, a specific anti-Faerie weapon. I look forward to debunking another wrong theory of Harry's on this scale probably about book 18 and at least one more during the BAT.)