What if to create a Starborn part of the process (or the whole process) is that an Outsider (or part of an Outsider) needs to be joined with a human host/embryo. It makes sense that Starborn can hurt Outsiders more easily if they themselves were part Outsider. Sort of like what makes them strong also makes them weak. If this is true, then it makes sense why people like Morgan were against Harry. Because no one likes Outsiders and being even part Outsider could be seen as a very bad thing to some people. It also makes sense why LTW needs to talk with a lot of people before telling Harry because he needs to understand how/what to tell Harry. Because just telling him (or anyone else), "Hey Harry, you are part Outsider. You know the race of being that no one likes. Well, you are kinda one of them because your Mom and others want to use you for the greater good." Harry would not respond well to that. (Yes, LTW doesn't talk like that, but that's the gist of the conversation). One thing I'm unsure about is how 40,000 kids could be part Outsider. I could understand a few because someone summons the Outsider and then bonds the child embryo to it but having that many seem unlikely. Maybe at certain times, the Outsiders can cross the border of their own accord and choose their own hosts. And there are so few Starborn left because some people hunt Starborn (or Outsider hosts) because of the fear and lack of understanding of what Starborn are or what they have the potential to become.

At the time of Harry's conception/birth, the Council could have been in favor of Margaret having a Starborn baby, but later, there was a political shift, and the Council's view changed. Do we know how long Langtry has been The Merlin for? Could he have taken over around the time of Harry's birth, causing the change in political mindset?

I've done a lot of rereading of select chapters recently but I've always enjoyed these moments:

In BG when Harry summons Toot and friends and Odin and other beings are scared by it
In PT when Freydis flirts with Murphy (as a Catholic I loved Murphy's response)
Morgan's trial/Ancient Mia being surprised/angry that Harry has a Foo dog
When Harry learns what Deamonreach is all about in CD

And so many more moments I get giddy over.

Another idea, the Walker called Mac "The Watcher," Mac may be "Out," but I wonder if he is still "watching." If I remember correctly, Mac only came to Chicago a few years before Harry, and Mac has appeared in almost every book. What if Mac is Watching Harry. Harry would not tolerate someone he saw or suspected was an enemy (white council or winter court) to watch him, and he would react badly (because it's Harry's nature). Knowing this, interested parties (white council, winter court, others?) planted someone in Chicago who Harry would not suspect as "watching" him. While Mac might not see Harry every day, he sees him enough to check his mental state and general wellbeing. Also, Mac knows a lot of people in town, and he can use them to learn more info. Also, people talk to and in front of bartenders like they aren't even there. This is another source of info for Mac.

Interesting idea about the portal. Only mortals can summon Outsiders so I suspect only a mortal could make a portal, assuming such things exist.

I would think it would be possible to create a portal. The gates are in the Never Never and so I assume the Outside is also part of the Never Never, thus you just need to create a portal to the Never Never.

Even if the fight subsided, ends, or there is a changing of the guard that doesn’t mean much personally to me (in regards to my OP). Sure all or many of the warrior class will be dead or gone but the support structure to keep the recruits and supply lines in tact is massive. Yes not all of these individuals are fighters but it’s Winter so everyone is at least a little dangerous. Also the different species/tribes won’t stop reproducing immediately it’ll probably take a few years or more before the population truly begins to decide. Winter is a lot more then just the Gates.

I was thinking, what happens after BAT? Assuming the "good" guys win and the outsiders are defeated, what happens with the Winter Court? The primary purpose of the Winter Court is to guard the Outergate. For this, they need hundreds of thousands of soldiers. What happens when they are no longer needed? Soon there would be an overpopulation on Winter in both the mortal and Fea realms. In the mortal realm, this would lead to Winter killing more mortals with the potential for a return to the Dark Ages (i.e., humans being afraid of the dark) or an all-out war between magical beings and humans.

I believe it was Lasciel who chose Harry, and Nicodemus was both curious, and concerned about it.  He found it curious, and it makes me wonder if Harry's mother had the same Coin.

What makes you think that Margaret had a coin?