Warning! Sorry, another author's note, but I can only respond to guest reviewers this way. And some reviewers make comments that others might be curious about what the answer is, so this way everyone knows the answer. Small amount of story after the hash marks.
A/N Sphygmus, let's start with your comments since you reviewed first. I've explored a few ideas in other fics as to what caused Xander to remember everything from Halloween including a natural psychic ability of some sort and as a gift from Janus in others. Your reinforcing color analogy isn't one I've thought of before, at least not in those terms, but in the Shadows au I do theorize that he remembers because Private Harris was another version of himself from another reality or timeline, a similar thought and in my Twisting Fates stories the thought was that a future version of him learned it and he tapped into the future knowledge, but only as far as what Private Harris knew as a soldier in general. There's an endless number of ways of exploring how Xander kept that knowledge since Joss let it go after Xander's arc was changed and we never got an in universe explaination.
In this story the Magic created the Sanctum, the spell to destroy vampires and Dr. Strange, who cast the spell and was guardian of the Sanctum. The Magic tied Xander to it, which reinforced some of the Magic knowledge from Dr. Strange.
I do agree on how Joss's efforts to get rid of Angel backfired, just like how another reviewer commented that hiring David and James to play vampires in the first place backfired. People couldn't see them as ugly, like the other vampires, which extended to why they couldn't see their behavior as ugly either and why they saw Xander's behavior toward them as ugly, they labeled him a bigot because he hated vampires. But Joss expected the audience to hate vampires too, including Angel and Spike.
Giles' allowing them to be together was acceptable to the fans because they also wanted Buffy and Angel together, but there's no good reason why Giles ever would let them be together, he knows it violates both the law and the Council's policies, therefore it follows that there's a bad reason. I've explored a variety of those reasons in other fics and have a new angle to explore in this one.
Starship Troopers, that one I'm going to have to think about, but mainly because I didn't like the movie. But the basic idea, Xander dressing as a character of some sort that's psychic and that being why he kept the knowledge, that idea is worth fleshing out in some way. New plot bunny! 😁
Mara, your turn. I'm not planning on Xander having any future knowledge, 'only' about seven decades worth of Dr. Strange knowledge from the comics. According to the wiki he debut in 1963, so originally Xander had everything from 1963 to 1997 in his head and the Magic extrapolated, meaning; Dr. Strange was a neurosurgeon and later a sorcerer and Sorcerer Supreme. The Magic initially gave every bit of that training and knowledge to Xander, including the medical knowledge, learned prior to the debut in the comics, though I didn't outright state that in the story, because for a couple of hours he was Dr. Strange, meaning he knew everything Dr. Strange would know. Dr. Strange didn't do investments, that came up several times in my research, so Xander would not have gotten any of that kind of knowledge anyway. Even if he did keep all of that it would be like reading the information on a computer screen, he had no real understanding of any of it, not the way he would if he actually learned it. Now he is going to learn it, for himself.
I'm not planning a crossover in this story, but you just planted the idea of a Xander/Marvel/Charmed Ones and now it won't let go, so I have no choice but to put it on my to do list. 😍😜
Kenn, I'm sorry I just saw your review. No, the vampires can't come back without god level help, the issue came up in the comics when a vampire from another reality was summoned and it was discovered, the spell was permanently in effect and that vampire was immediately destroyed. In Buffy they were initially created by an Old One, it's possible an Old One might be able to recreate them, but that won't be happening in my story. The whole idea was for them to be destroyed permanently. And the Hellmouth is currently defunct as well, of course there is one in Cleveland (which Cleveland is still a question). 😗
######
Angel cautiously stepped out into the sunlight, even with a reflection, lack of strength, disgust at the smell of blood and feeling of his own heart beating in his chest, the sunlight was still a possible concern. Slowly he became aware of the warmth of it and yet he wasn't bursting into flames. He let himself enjoy a forgotten pleasure for several minutes, before reminding himself he needed food, real food and headed for the grocery store. He'd never been in one before, Liam's memory showed food was delivered to the house. Angel was confused, a feeling he didn't like, at all and had no idea if any of it would even taste good, not to mention, how did he cook it? He'd warmed blood in a microwave but that wasn't the same thing. After standing in the grocery store for several minutes feeling very confused and lost looking a woman approached him and asked if he needed help with something. Instead of answering, Angel just left. He hated feeling so lost and confused. What the Hell was he going to do now?