Luccio also talked about possibly training up her new body to her previous ability. Possibly magic use is part physical and part mental - nerves, perhaps, conduct magical force, without training they can only do so much. Luccio swapped into a non-mage and had SOME ability. Maybe Corpsetaker is limited in the bodies of non-mages, but she can pull her one specialty. Aren't the Black Court eager to get wizards to turn? Maybe it is not just mental. Unfortunately for Luccio, looks like the little cutie she inhabits just isn't going to be able to get where she had been. Sounds a little Heaven Can Wait (Beatty version) ? His new body had to be trained up to get NEAR his old one.   

And summoning is not necromancy. No death involved.

Nor is body switching like the Corpsetaker does,necromancy, she doesn't bring anyone back from the dead to do it.  If I remember correctly, Luccio is limited in her new body, while she can still do
the wizard thing, her ability to forge swords to match her new wardens is gone.  She also found that her young new body retained all the urges most healthy young bodies have.  It is one of the reasons why she was so vulnerable to Peabody's ink and could be manipulated into making a play for Harry.

Really? every Necromancer Harry has met has commented on his ability in this area to some degree or other and in Battle Ground just before he takes on Drakul and seven Necromancers Harry comments that four Necromancers nearly wrecked Chicago before, five if you include him, a neophyte who still managed to walk off with best in show.

One robin doesn't make the spring nor does one T Rex make a necromancer.

Since Odin is a god, I don't think he fits into the necromancer category either.

As it stands it was Harry and not any of the other four Necromancers who actually summoned the Erl King, he has an intuitive ability in this area as to what would work best and where, and has a considerable reach. He has summoned the Erl King, Titania, Molly, Mab and Mother Winter, all strong enough to avoid a summons (but admittedly in at least half of those cases Harry being Harry and thus intensely annoying may have resulted in the summoning being successful, e.g. Titania hitting Harry with a lightening bolt.)

All of the above listed were alive when summoned, successfully or not, necromancy had nothing to do with it.

Margaret couldn't bargain using something she didn't have and didn't control.   Lea would be a fool for taking any such bargain.

Almost certainly she hid Margaret and given the timing, whatever deal she had died at birth.

I don't disagree with that, I also think it is even more complicated than we know.  I wouldn't be shocked if Mab was even deeply involved than we know in the conception of Harry.  I think the bargain goes further than the usual "first child" in exchange for protection thing.  Mab thinks in the long term, she knows her job is to protect the Outer Gates and that there was a lot of deep pooh on tap for the future.  I also wouldn't be shocked if Rashid had something to do with this as well.  He knew Margaret well, they were friends and together worked with the Winter Court.  I am willing to bet that neither he nor Margaret fully trusted the White Council to do what is needed for the coming BAT.  When Malcolm came along, it was agreed that he had the elements they needed to contribute to a star child.  Margaret knew she was a target as soon as she left Raith, and being pregnant made her even more vulnerable.  So the unprecedented step to make Lea the godmother to the child was taken, in exchange the child would be the future Winter Knight. Rashid also pledged to protect the child from the White Council, which he has done from time to time.  Yes, Morgan pledged this, but he also had second thoughts, pursued Harry and in some ways made things worse.  Rashid is more under the radar, taking Harry's side in front of the Council, but not overtly, instead as a powerful voice of reason that the Merlin and other member find hard to contradict.

Margaret had previously had a child, so she was not a first time mother, so a shorter labour was likely. Hitting a particular day would involve her using the ways temporal effects to get her due date to Halloween, most likely delaying labor, so she ‘overshoots’ her due date. From then she would need a doctor to induce labor, a little bit of Earth magic to squeeze the baby out with a focussed gravitational field (her father and her son both did that spell) and Bob’s your uncle. The child is Starborn as soon as it is independent of Margaret, and the entropy curse bites on Margaret and she uses her Death Curse on Raith.

Just because her labor time might be shorter, that doesn't make the timing any easier.  Here is another question, Raith managed to locate her for the entropy curse, was he trying to kill both mother and child? Apparently not, why?

Which is so strange, I think, as celestial movements can play out over much longer periods of time. But I do think you're right in that it's only a special hour or so that happens, and that's where the 40-50 thousand number comes in. That being said...it's surely not just births, as Jim hinted that you only need to be in the right (wrong?) place at the right time (which Jim thinks makes you really unlucky). Being born is by no means the only way to become a star born it would appear.

Or considering the population of the earth, it could be millions born in the special hour.  So it stands to reason that it is a lot more complicated than it appears to conceive and give birth to a star born.  Lash hinted that it was to Harry, she made it clear that it took a lot of planning on Margaret's part for it to happen.  It is also clear that in her choice of Malcolm to be her star child's father Margaret was trying to break the mold for the norm of what a star child is.

  Immortality doesn't appeal to everyone though, and at this point anyway, I don't see that
it appeals to Harry all that much.  It was a possible selling point for accepting Lasciel's
coin, and he didn't.  There is no joy in immortality if all your nearest and dearest are mortal
and age and die around you.  Friends and relations are important to Harry.   Immortality isn't a substitute for intelligence, but one can hope that with the added centuries some wisdom is gained.  I think she has been around for so long and in a position of power for so long that Mab no longer understands what Eb was trying to say.  Also perhaps like a lot of highly intelligent people for whom things come easy along with power, she has no understanding for ordinary mortals save for how to exploit them.

This makes me think that it is easier to gain immortality through a Mantle rather than through any other means.

I think that is the only way for an ordinary mortal to gain immortality.  Wizards naturally live a much longer life span that vanilla humans, but eventually even they grow older and die.  Denarians are the closest I think to ordinary humans gaining immortality as long as they keep the coin.  The Holy Knights are mere mortals like the rest of us, even if they remain Knights for years like Michael and Shiro, eventually get old and will die someday.  I'm not sure whether or not White Court Vampires are immortal, they live a very long time, but I don't remember it being said they are immortal.  I don't think either the Summer or Winter Knight mantle gives immortality along with it, because the Knights remain mortals, they are not transformed into Fae like the Queens.  So in the case of Molly, she gained immortality with her Lady's Mantel, but the price for it may be her soul.

Peace Talks page 271 Malcom is wearing a Chicago Cubs baseball cap, indicating it is is hometeam. The same memory sequence has Malcolm doing gigs in Colorado and Ohio, suggesting my earlier premise that Malcolm’s home base was Chicago and that he worked the Midwest is valid and that Harry was born there in Cook County Hospital is also valid. This may suggest that the Mug from Christmas Eve was actually stored by a friend/theatrical agent of Malcom based in Chicago and that there is a hoard of Dresden memorabilia to be uncovered by Harry.

It could all be as you say, but I prefer to call it a Christmas miracle..