As for my own musings on the Fellowship of Truth… let me put it like this. I know settings that could conceivably have enough power to bully Aum in a crossover event. Where phrases like "Hubris is a cowards word" is the order of the day. The Fellowship of Truth is not from any of those settings(much as they might like to be), nor do they have any connections to such settings they can exploit. And it's a good thing too.

Being the only people with instant death spells they could cast at people they wanted to go away and only having to fear each other for as long as they did(http://suffadv.wikidot.com/forum/t-7001576#post-3919600), combined with whatever metaphysical sacrifices they had to make to learn their magic, could depending on what kind of interpretation you take of the Fellowship, lead to them having mindsets wildly divergent from a typical humans concept of ethics. The Gift made death spells obsolete. Meaningless.

Combined with the cosmic atrocity that afterlife sorting is random while hells exist(Though I consider fixing a problem like that possible for Player characters who figure out enough about the Worlds Beyond to get Aum therapy, you could make an entire campaign of that scenario for Soulpact PCs.), the fact that they're trying to undo the gift anyways, condemning who knows how many people to eternal torment, makes them monsters on par with the Slaughterhouse 9 from Worm or possibly even worse. No matter what they say, no matter what argument they make, if afterlife sorting was random before the gift, from a Utilitarian standpoint they are monsters and the gift was an objectively good thing

There are crossover rules for Sufficiently Advanced. It's possible if you want it to happen. Sufficiently Advanced has different Metaphysics from Sorcerously Advanced(Page 10 Interregnum Edition), and multiple settings that could conceivably be viewed as separate universes(Powder Keg vs the Divide for example), leading to Sufficiently Advanced being a separate multiverse with diverging metaphysics, because the Patent Office's universe doesn't have to be part of the Kaleidoscope and the Sorcerously Advanced Multiverse(Presumed Multiverse, page 215 of the interregnum edition introduces the possibility of doubt).

Page 254 Interregnum Edition

This
section is for folks who want to run a
game where characters from SA2 have to
deal with the Great Basin, or where the Patent
Office’s universe is part of the Kaleidoscope

Character hybridization is canonically possible. Aum brought a style of magic(assuming Aum created it and isn't just tweaking some hypercosmic mechanism) into being that in spite of its dependency on environmental flow, could conceivably remain somewhat operational in an entirely separate multiverse with diverging laws of physics/metaphysics. This gets into the territory of the kind of Versus debates that usually get very stupid very fast due to arguing about levels of infinity. Considering one of the lore options for the size of the Kaleidoscope is infinite… the highest end interpretation of Aum one can conceivably get from Sufficiently and Sorcerously Advanced for versus debate purposes is either infinite multiversal(if you look just at the biggest possible size of the kaleidoscope, and assume that all the Aum titans in all the universes are extensions of the same higher being), or Megaversal(Defined as a collection of multiverses with divergent metaphysics and events(edit: as opposed to just alternate histories of the same universe with a single point of divergence or something) and operating under the assumption the founts and the sources of flow were set into being by Aum). So your Crore Stones Plan managed to hit Aum. Aum is an infinite multiversal or megaversal being under high end interpretations. All you hit was his finger.

Fuck the Fellowship of Truth is what I'm saying. :)

This horde of unread, filthy, smelly peasants must be stripped of their magic so that Magick (note the capital M, note the K!) can once more be the sole preserve of the deserving, be in the hands of responsible, trained and educated wizards who have had a proper apprenticeship and everything. Well, good luck with that, Voldemort.

So what I'm saying is that the members of the Fellowship of Truth are basically magical fanbois, purebloods to use JK Rowling's term (I'm trans and she has greatly disappointed me). They have that sense of entitlement. There's a reason that the term muggle is so often used in many fandoms to describe non-fans or newbies. Thus the behaviour of toxic fans in real life and elitist wizardly cabals in fiction can be used as approximate models for the general mindset of the Fellowship of Truth.