Not so sure I agree that Margaret was intended to be used by the White Council, but it's possible. I think she rebelled for far more ordinary reasons though, like having an authoritarian mentor/parent and disliking how the Council operates and bullies others...just like Harry. Ebenezar and Luccio effectively tell us this anyway.
The evidence for that is in my opinion is the fact that she apparently knew the significance of on and how to conceive of one. Now it is possible I guess that it was Lord Raith that was pushing her for this. Wouldn't he have loved a star born child that he could use as a weapon? Provided of course it was female, a male child especially a star born one would have been seen as a threat. So I don't think that happened, and while the idea of star born children may be general knowledge, the details of how to conceive one is another matter. Apparently the Senior Council has this knowledge, Eb wasn't a member at that time, but as Blackstaff with the job perhaps of taking one out from time to time, he'd need the knowledge. I think the real reason the Merlin wanted Eb to observe and possibly wipe out young Harry wasn't because he feared he'd revert to warlock, but because he knew Harry was star born. And just how did that know this? I mean they may have had Harry's birthday, but did they have the other information about him? That tells me they knew very well what Margaret had done, and had been a part of it at one time.
I think it must be difficult to identify star born up to a point...similar to how Rashid even struggles to detect Nemesis (or Uriel for that matter). Obviously people know about Harry now that he is starting to use that power, but I wonder about others. Because Elaine could be yet has seemingly remained undetected by the White Council (although not by the Black Council perhaps). Obviously Outsiders know them, considering the power star born can wield against them. But I wonder how easy it is to find them? To identify them once found? Particularly if they haven't been using their power.
Is it? I mean several characters have come out and called Harry a star born without much trouble. Harry doesn't have a tat too on his forehead proclaiming it, so without additional information how did they know? Now HWWB didn't seem to have any trouble, though he didn't come right out and call him a star born. Do Outsiders make a habit of visiting and checking out possible star born confidantes?
4. It could also be a prophecy-type deal that once the person is told, it certain events in motion, and maybe everyone is building up their forces and defenses to ensure that they are ready for what comes next. Several beings have hinted they want as much power as possible when things kick off. Titania also implied to Mab that some sort of threshold had been crossed, a milestone that means those very things are about to happen (relatively speaking).
But Harry has always known since White Knight that he was different because of how his mother and father conceived him, he had power over Outsiders. Also from the soul gaze with Thomas, Margaret told him that it might be unfair the burden her and his father placed on him. Also in Harry's vision/dream of Malcolm, he repeated what Margaret said. As late as Peace Talks Eb told Harry he can deal with Outsiders, yet Harry hasn't used that knowledge to go Outsider hunting.
So he knows quite a bit, but unless there is some more to the huge reveal, why not tell him? I think a lot of people will have a lot to answer for once Harry knows, that is why no one is sitting him down and telling him the whole truth.. I include the Winter Court in that, I think they had a lot to do with why Margaret ultimately made her decision.. And
that'swhy Harry is really going to be pissed at Lea for becoming his godmother, he is going to be pissed at the whole Winter Court.. Hell yeah, he will feel used and abused in the typical fashion that only Harry Dresden can feel..