Very good points.
I think the question is what was going on when she escaped the White Court. She was apparently addicted to Lord Raith, and whatever she was intending to get out of her plans with him had probably gone in the toilet (not least because Ebenezer refused to cooperate). Making a starborn baby doesn't seem like the kind of thing you come up with on the spur of the moment or when you are in that kind of situation, on the run and cut off from your resources.

It's not so much a factual thing, as the sense I get from the way her relationship with Malcom has been mentioned, as a kind of surprise thing she stumbled into, with a legit good man, that opened her eyes to what she was missing in her life with all her political ambitions and her schemes with the vampires. It just doesn't feel like the kind of thing she'd go into to make a paranormally special baby. Also, I think Malcom would have had issues with, or at least questions about, her disappearing and an abnormal length pregnancy.

I would say that she wouldn't. It's not just the date, but a lot of other stuff that has not been specified, and is probably beyond human ken, and also the kind of thing that could not have been predicted, and something they can only realize after the fact.

Well, that's how I reconcile it, that it's NOT the kind of thing you can force or make happen, that the timing and place and whatnot of the birth, the factors that could be arranged, are not the entirety of it, that it depends on magical things that can't be anticipated or controlled.

Also, I think it fits with my headcanon of Margaret's situation. She grows up the daughter of one of the Council's heavy hitters, a guy who was, or had been, leader of the Wardens, she's naturally talented & brilliant, which makes her cocky and overconfident, she thinks that she knows enough to change the fundamental workings of the Council, she makes her connections in Faerie, maps the Ways through the Nevernever, and as a result, gets accustomed to the idea that she can duck any consequences, because not even the Wardens can keep track of her, and can get away with anything. Then her intrigues get her involved with the White Court and Lord Raith, and she tries to do something related to the Red Court and get her father involved, she ignores his warnings and ends up a plaything of Lord Raith and bears him a son. She has to flee, leaving behind her son. The woman who thought she could do anything, can't even protect her baby, because she is addicted to his father, and hers was right after all. And at her lowest point, she meets Malcom, falls in love, and marries him. This seems like it would break her addiction from Raith, given what we are told about the effect of love on their power.

It feels like the right beat in that story is that Margaret, at this point, wants to do it right with Malcom & their forthcoming child, she's done with the politics and schemes and everything that made her reputation among the wizards and the rest of the paranormal world. She might have had to leave her first son stuck in the middle of that, but she's going to keep her child with Malcom safe. And of course, the twist is, the joke's on her, she can't outrun her past, as her enemies catch up with her, and she can't save Harry from her world, because it turns out that he's a starborn.

I think this is a more fitting story in the series than some long-term scheme by Margaret coming to pass. It's all just a big mess, and parents fuck up their kids even when they are trying to do right by them. This is just my intuition from how I am reading the underlying ideas of the series. It's not a strictly evidentiary deduction.

For the record, I believe that Maggie was not thought up for Changes, because the time period where she would have been born is during or around Blood Rites, the most family-themed book in the series. If there was any other way to drop a hint, I think Butcher would have, but with the limitations of the point of view structure, and the established facts of Harry's character, there was simply no way to do so.