Themes of control have been noted, but here's another one: Our enchantment makes things malleable, which could allow them to be fixed... or broken, in unwise hands.
Also this enchantment makes me want to offer to teach Hiroko. She does have artistic aspirations, even if sculpture isn't exactly her field.
Well, from what I've seen, Firn does try to keep things canon compliant. It's always possible that there was enough material in place for Firn to adopt or warp the rest of Wraith arc to fit in with it later.
And as Firn has repeatedly said, he does like it when there are layers to things.
Yeah, the original theory post itself is trying to be all-encompassing, but there's a number of possibilities within the sphere of "wraithbrina." Give me a moment with the centrifuge to spin them all out.
1: Sabrina is magically/metaphysically a wraith. Essentially, "Sabrina's wish gave her wraith powers".
As you've noted, this is the simplest and most likely version of the theory, since it is pretty close to the observed evidence after all.
The most relevant question here is: Do we merely imitate wraiths? Or by making our wish, have we truly become part of that system?
Another interesting question here is, "If Sabrina's powers serve as a partial wraith-esque response to Madoka's wish, then does the fact that she only controls grief mean that other magical girls might also be out there with wraith powersets?"
*Stares pointedly at Riona Mag Aoidh*
2: Sabrina is physically a wraith.
The idea of Sabrina physically being a wraith pulls a lot more from Wraithpurgis Gretchen than it does any of the wraith!meguca. Yes, the latter are wraiths subverted by grief only appear human through memory manipulation, but the former is notable in that we see that it will lose its wraith form entirely during the explanation of it in chapter 8.
The idea here is "Okay we've seen what enough witch power will do to a wraith, and we see wraiths with more human emotional cores pumped into them gaining more human outlooks, so could you pump enough humanity into a wraith to make it turn flat out human?"
Ultimately, there's a potential path there, but I haven't seen any real evidence it was taken.
3: Admixture: Madoka's wish to fix everything produced a different kind of wraith as a response.
It was brought up before but the core question here is worth repeating again: Are wraiths and their powers consistent, or did they form as they did thanks to the exact nature of the universe altering wish they responded to?
As noted earlier, canon wraith power and motives are very similar to canon Madokami's powers and actions. After all, if destruction of witches before their birth is a core universal phenomena and wraiths are part of the system that helps manage universal phenomena, then it makes sense that wraiths might also pull some of their power from that same universal principle.
Likewise, wraiths don't just go after potentials, they also go after magical girls. By my own reading of wraith arc, just as you can apply their stated reasoning to stopping new magical girls from overturning the laws of the universe, you can also apply it to them stopping witches from being created and so going against the law of cycles. To me it seems they they talk about the dangerous of emotional energy to the world just as much as damaging the universe.
That's consistent with them trying to assist the law ordained by Madoka's wish while also preventing something like it from occurring again and the idea that a different wish might have provoked a different response.
Which leads to...
3a: Madoka's wish (partially?) subverted the wraith system.
We're going with the idea that wraiths are either the system that brings the universe back into proper alignment after a wish or a manifestation of it. So if the definition of proper alignment changes, say, by someone rewriting the universe to redefine what "fixed" is, then the resultant wraiths will bring things towards this new state.
And if canon Madokami might have partially altered the wraiths via her wish then PMAS Madoka-1's wish to ensure everything could be fixed could have entirely upended the universe's end goal.
Which could lead, one way or another, to a variant wraith appearing as an agent on Earth in order to set that in motion. Assuming all of the above, it would make perfect sense for it to not have the wraith canon power set, because it's the result of a different wish. It might even come into existence vastly differently.
Instead, if the wish determines the wraith, we'd expect to see a set of native abilities based around having the power to fix things, or at least make them fixable. And, of course, it can be hard to fix human problems, without a human outlook. Beyond that knowing what problems existed to be fixed might also be natural.
True. Working further however, even if there are many odd states of normal matter, it may be that grief only exists in a number of primary states. What a material does depends on its properties after all, and there's little reason to think that grief's fundamental unit resembles the atom. (For one thing creatures made from grief are usually invisible to normal humans, grief we control is the exception and we infuse it with magic.)
For instance, plasmatic grief might not exist, as plasma does sort of require ions, while grief, if anything, might be considered already ionized: It's only one emotional "charge" out of a much larger spectrum. Or likewise making it grief plasma might involve removing our grief controlling magic from the equation.
Which might instead suggest a superfluid or Bose-Einstein condensate analogue to be the fourth state. I'd going with superfluid specifically since it's normally found in extremely cold environments, and in the inside of neutron stars, both of which one might associate with Moksha wraiths. The first for the obvious elemental coldness comparison but also neutron stars due to the end of the universe/Wraiths as metaphors for universal progression, comparison.
Plus there's an additional comparison with Bose/Einstein Condensates/Superfluids: The matter of all atoms act in concert, temperature wise sort of looks like all emotions flow in concert which could be likened to achieving a higher state of consciousness. Or something along those lines. (I'm sure a proper physicist could make a much better case of drawing a connection between states of matter and enlightenment.)
(Note that as both noted states of matter described here frequently occur in relation to extreme energy states, I'd recommend we begin testing any weird material states with a single small marble, at distance.)
Looking at Wraith Arc, I'd say they're whole collections of emotion and memories, formed and pelletized. Like what we do with iron ore... and some uranium-dioxide nuclear fuels. (Yeah the conversation has long since moved on, but I wanted to get the obtuse angle on the "untrained light-water-reactor technician" crack out there.)