One theory about Wanda and Pietro is the stone didn't 'give' them their powers at all, it merely unlocked the potential they already had (they could retroactively be the first MCU mutants). This would explain why no one else survived the experiments, yet both of them just happened to do so. As a theory, it could fall apart when taking Captain Marvel into account since she's the only other person we know of so far who got permanent powers from interacting with an infinity stone and we've been given no reason to believe her powers are anything other than that. But, in the comics the Captain Marvel powers are actually a sort of Kree legacy and Carol finds out a long, long way into her life that the reason she was actually capable of receiving them from Mar-Vell was because she actually is half-Kree (her mother was a Kree exile hiding as a human on Earth). So it is possible some version of that concept could turn out to be true, as well, in the mcu. If that happened, then they would all have just had their pre-existing potential unlocked, not been granted Infinity stone powers based on the specific stone they interacted with. But... that would require Carol's powers to actually be a pre-existing legacy of some kind outside of herself, which doesn't seem to fit with what they want for that character. At least not based on what we've seen so far.
I've long held the theory that there's something about the human genome that's predisposed to getting superpowers. I don't know if 'Eternal's' will touch on this but I'm vaguely aware that in the comics some celestial, or cosmic being monkeyed with early human genetics (also Kree genetics, IIRC?) So assuming that's the case in the MCU too then you have a number of pieces of evidence to support the idea that modern humans, to some degree or another possess latent markers of whatever it was that was done to their ancestors.
First off; Extremis. It's explicitly said to exploit a "slot" in our biology that appears destined to receive an "update". Since that is in no way how natural selection works, the only way something like that could exist is through artificial engineering.
Secondly; Inhumans. The Kree experimented on any number of worlds trying to engineer various species with varying degrees of success...except on Earth where the results were off the charts. So much so they lost control of Hive and had to kill the project and never attempt it again.
So basically, all super powered humans are an expression of this same latent engineering being activated. Extremis being incomplete and unstable produces mixed results, mostly of the pyrokinetic and regenerative variety. Teragen is seemingly a total dice-roll. Erskine's formula plus "vita-rays" gets you a super soldier, or a Hulk/Abomination if you add gamma. Though even Erskine himself said the transformation would be a reflection of what's already there, it's ambiguous just how literal that was supposed to be...but considering what happened to Banner and Blonsky probably more so than not.
As for Carol, a tesseract powered device didn't just blow up in her face, it passed it's energy directly into her, and then she got a transfusion of Kree blood. That double-whammy appears to have been what allowed her to survive the process and gain powers. Perhaps something similar was done to Wanda and Pietro? Like it wasn't just the mind stone, but the mind stone plus an injection of *something*? Maybe List got his hands on some GH.325 or Centipede serum from Garrett, maybe it was some last trace of the Erskine formula (between Rogers, Banner, Blonsky & Sterns there was no shortage of donors), or maybe it was teragen, something left over from Jiaying, or something new entirely.
One way mutants might fit into all of this (assuming Wanda doesn't just 'House of M' them into being) is that between the aforementioned hereditary engineering, Earth being host to at least two infinity stones for about a thousand years, and later being ground zero for the snap and the resulting in a power surge of ridiculously cosmic proportions that nobody has ever seen before; it's possible a lot more random individuals with latent powers are getting activated than ever before. Only now it's not weird mad scientist experiments (human or otherwise), it's seemingly spontaneous mutations...
The whole reason he's on site to begin with was because he was supposed to come down and make contact with the witness, right? It would be rather ridiculous for the guy who's job it is to go and find the witness to not be allowed to know who exactly he's supposed to be trying to find.
True. But like I said, none of the male "residents" are really throwing up any red flags, at least not like Agnes. If she's a witch then it's possible she had cast a glamor making herself appear as someone else entirely, hence the gender confusion. After all, that was
a signature trick of Loki's, who learned it from Frigga, who was raised by...*checks notes*...witches! So there's precedent.
Also, what are the odds that a witch, an FBI WITSEC subject AND a crazed Avenger all show up in the same arsefeck nowhere town?