I'm late to commenting on the OP, but I want to thank Neuro for laying out this thesis and its relevant details. However, there are some questions that I have and a few comments I would like to make, but some of the comments are so minor I may just do them in a separate post.
If I understand the OP correctly the proposed plan by Maggie and Lord Raith was to create a Startborn with the ability to oppose the Outsiders; and possibly unnamed others, for the benefit of all the major powers on Earth, be they mortal human wizards or vampires, and the supernatural powers of the Nevernever as well. However, while Margaret LeFay and Lord Raith openly proposed this plan; possibly with the support of Justin DuMorne, the truly major supernatural powers; represented by Uriel, Odin and Mab, have been working in a covert fashion, and generally speaking their efforts have been indirectly linked to one another, rather than a formal alliance with distinct roles spelled out by an agreed upon treaty. Because the truly great powers have foresight, they don't really need to talk to each other very often. They know where their individual strengths lay and what their counterparts are likely to do as well.
(Boy is Harry going to be pissed if he finds out he is something of a group science experiment; though Lash kind of hinted to him that this might be the case in their final discussion in the Raith Deeps.)
I'm not sure why Maggie and Lord Raith would have to announced their plan or why they needed the agreement of the Red Court or Ebenezer McCoy. What part were these two parties supposed to have played in creating a Starborn? However, I think it is easy to see why different factions would have wanted to get control of Harry or kill him.
If one party didn't really understand or believe the threat the Outsiders represented, then it might seem like a good idea to kill a Starborn wizard before he becomes the White Council's newest super weapon. The reverse applies if the WC doesn't believe or fully understand the threat, and instead see a Starborn wizard as a super weapon who has been created as a tool of other dangerous supernatural powers.
I can understand why Lord Raith would want to create a Starborn wizard, especially one he could control. He might have been protected by Outsider magic or some variant of it, and he did use human summoners to call up HWWB, but he could have thought that having a Starborn working for House Raith was a way to balance the scales between himself and the Outsiders. In this way LR couldn't merely become an Outsider puppet, he could be their equal.
If the OP is correct, or at least close to being correct, I would hypothesize that Margaret LeFay figured out how to create a Starborn child without assistance from the WC (Ebenezer), the WCV or the RC. This would explain why LR didn't try to track down Harry as a child. He didn't realize how important he was, and not just important to the curse which Margaret placed on LR.
I want to comment on Lea, Marcone and Nicodemus, but my observations aren't relevant to major idea of the OP.