Also, a little bit of the Erlking is in every weapon used to hunt, so every kill gives him more power.

Is that confirmed in the books or WOJ? If it is, that really is interesting especially if you consider the hexenwolf belts and whether they were created by the Erlking or not.

I don't recall anything about Simon having a female apprentice?

We know that Simon had a female apprentice named Larisa Yevtushenko from both the paranet files and I think from either one of the books or a WOJ. I can't seem to track down the book quote at this time though. She is mentioned directly in an excert from the Paranet files at this site though - http://www.evilhat.com/home/paranet-papers-sneak-peek-novgorod-4/

I really like the Cowl is a formorian (or scion of said race) theory considering some of the subtle clues that have happened whenever he is onscreen. One particular discription has stuck with me recently since I did a re-read of Dead Beat:

Quote from Dead Beat:

Cowl's fingers formed into a rigid claw and he snarled a word I couldn't quite hear, slashing at the air.
There was a surge of power, darker this time, somehow more nebulous. The air around them blurred, there was the sudden scent of mildew and lightless waters, a sighing sound, and as quickly as that, they were simply gone.

The use of the words 'dark' and 'nebulous' to describe the feel of his power, as well as the description of a sudden scent of mildew and lightless waters suggests dark watery depths. Sounds quite formorian in feel to me. It's not definitive but it sure is suspicious when added to some of the other evidence provided.

This doesn't neccessarily discount Cowl being Simon as it might be that he made a deal (Faustian bargain anyone?) with the Formorians for power and they may have altered him physically as well which would account for his warbling non-human voice and how he appeared physically in White Knight.

Lastly Cowl's power has been described by Dresden as being 'cold' but not greasy on multiple occasions. Since during Dead Beat Dresden was dealing with multiple necromancers, he may have just assumed that Cowl's power was also necromantic in nature as apposed to something else that felt very similar (ie formorian flavoured power) just because that's what he was expecting and the power pretty much met all the prerequisites for necromancers (cold, dark, and opposite of how his own power feels). Funnily enough it seems like Cowl didn't actually do any overt necromancy that I can tell during that book other than the ritual at the end. His other ritual was to take out the power in Chicago which seems more neutral than necromantic to me.

Still it's just a theory.