Just an observation. Somebody was looking for special children and using the courts and other institutions to find them. This has been a recurring theme in the books. Elaine was probably picked specifically because she was similar to Harry.
The White Council often seems like they wouldn't be much good navigating the mortal systems...but they clearly have the wealth to have multiple law firms on retainer. So they are one possibility (or at least someone who was White Council, particularly if they were younger i.e. Justin).
The Fae are supposed to be good at navigating the legal system - Mab herself according to Jim is a very capable attorney.
The White Court also have shown to be able to leverage the mortal systems to their advantage exceedingly better than most other supernatural nations. Given the White Court's association with Margaret La Fey, and their association with Outsiders/the Game, they also are excellent candidates.
Hard to say who it really was though.
We don't know about Cowl, if it weren't for Bob, Harry, and Sue, he would have pulled off Darkhallow.. So is Cowl starborn? What happened to him anyway? He hasn't been heard from since White Night and had given the impression of being a major player.
I think being blown half-way to Hell probably has kept him side-lined. Given his extreme likely hood of being Black Council or whatever, and how much they are behind the scenes movers, I'd say that's reason enough to not be seen. Although I do miss the character. The Doylist reason is probably that Jim just couldn't fit him in the other books, his character isn't needed for the story so far.
There is a certain level of Butcher being Butcher in all of this. My mother could be Starborn for as much specificity as he gives. But obviously birth date isn't sufficient in and of itself. Where you are when you were born seems to be important. So the date is knowable but the location may not be and the only way to know is to test for it. Or maybe Margaret figured it out. Which might explain why Malcolm wasn't at the birth.
If you want to go all tin hat, the fight with Justin may have been that test. Put Harry on the run and see what happens when he crosses paths with an Outsider. If he's Starborn he passes, if he isn't, he gets eaten. Easy Peasy and here is your guaranteed Genuine Starborn Certificate.
True enough, frustrating as it is. I think the location is extremely significant. And I would bet dollars to donuts that it's Chicago that's the place. No idea how much. But given that there were 40,000-50,000 star born I'd say it was anyone in that area. I'd assume it was night time. It's hard to imagine that many beings all rounded up and prepared but it's possible. Most rituals of some sort of blessing/transformation require a preparation of the thing being changed - so I expect Harry (and whoever else) might have been prepared earlier. The way Ebenezar puts it (and the number given) seem to suggest it's all those born in that area, at the right time. However. As we know being mortal isn't a requirement so obviously some beings are turned into star born after being "born" - perhaps long after that.
I think He Who Walks Behind was the one running the operation, not Justin from what I remember? I suspect the test was more about whether they could make Harry more of a monster - the hint from Lea about how it's so difficult to teach a young wizard to use magic to destroy as it's so delicate.
The difference between Vadderung and Drakul is the Kringle Mantle, which like the Queens renders him immortal not merely ageless and extremely difficult to kill, like for example the Redcap. Otherwise they would be on a equal footing. The other difference is that Odin has access to Soulfire and this is required to create Einenjharen, the only way it seems to safely resurrect the dead appears to be Soulfire (e.g. Jesus and Lazarus). Drakul’s son created the original Black Court Vampires, perhaps in response, using Hellfire?
I am not so sure about that. I think Kringle is immortal but not Vadderung, and so being Kringle is what keep Vadderung ticking along. I think it's an important difference myself.
I think Odin got his Soulfire from doing a favor or several for Heaven. Drakul created the original Black Court Vampires, Dracula (Drakul's son) grew them into a Court and made them famous. But I do like symmetry of using Hellfire to create the Black Court. Although I would point out that Hellfire isn't a creative force...but maybe that doesn't matter. The other thing that can raise the dead (more than just ghosts) is Necromancy. It seems to be more of an aberration though. That said, apparently the Einherjaren are the result of Soulfire and Necromancy according to Jim.
Also, don't all angels have Soulfire? I believe Bob says that much. I am sure it's very rare to get Soulfire if you're not an Angel but I can imagine that more than Harry and Vadderung might have it, and I doubt they are the only two ever apart from Angels to get Soulfire.