That you're awesome. 

The biggest problem is Lash: we have a WOJ saying Lash was in GS. Who can it be except the parasite?

Why can't it be both? 

Demonreach growled.  In all capital letters.
And the headache vanished.
One second, my scalp was tightening up as two separate ice picks dug into my skull in the same places they always did, and the next the pain was utterly gone.

So, just to play devil's advocate, here's the theory.  HWWW did indeed infect Dresden at some point in the past, but has always lain low.  Similarly to Lash, it could have made itself known to some portion of Harry's subconscious (or not), but it's smart.  Harry's important for some reason, and the last thing it wants is to be discovered.  Lash commits quasi-harry-karry (misspelled on purpose), leaving room in his mind for the Nemesis to take further root.  Previous headaches were initially caused by Lash doing...whatever, but the Nemesis sees an opportunity to inflict pain scot-free, and makes its own head-spike thing, and is smart enough to stop every time Lash does.  It's not perfect, but I could see how it might work.

For two, soap.  It sounds silly, but honest to god, most of the other guys who work in my office know when a female sailor has come through within the previous few minutes.  My sense of smell is shot, otherwise I'd be in the same camp.  For whatever reason, anecdotal information seems to suggest that women use different soap, or that the smell lingers on their skin more. 

Kali, if you're still looking for someone by the time you're ready, drop me a PM.  My current work load is, shall we say, very light and I'm a captive audience, so I can get you a turnaround of 3-5 days.