I'm gonna double-post too, since I feel I had a natural break, and the possibility existed (though I guess, not at this hour), someone would have replied.
Of course all of this raises a deeper question: does Cassandra fully appreciate what she's risking? For all the ill that comes before the end, Patroklos and Pyrrha did achieve something pretty monumental: they seemingly destroyed or at least sealed away the two spirit swords for good: and by the end of SCV, it's clear that both weapons are a bit of a blight on human affairs and that seemingly you can't have one without the other turning up. Cassandra is risking undoing humanity being done with the swords. Which makes me wonder...maybe that's the point? Elder Cassie is looking distinctly malfested--or maybe combo possessed by both swords, which are using her as a conduit to undo being given the boot into the Astral Chaos?
I feel like I've addressed most of what's all in your first post already, but this I will respond to. I do believe it quite possible that Astral Chaos Cassandra was being manipulated by the two swords, fused after the events with Patroklos and Pyrrha, may have been banished to Astral Chaos with Night Terror (since we haven't really established what happens to the sword spirits while they are dormant), which could be a level of deception that Cassandra is intrigued by, but ultimately realizes is not worth the cost, and lets things pan out as they were, for the greater good. Though I'm not sure about "for good", because Siegfried had already done this before, by forging the Soul Embrace. It was broken by Zasalamel. I don't believe they can ever be gone "for good". (I mean, if they could, that would mean the death of the series... do we really want that? No swords, no games.)
So what was up with the mysterious person or entity offering Cassandra strength? So here's a little bizarre bit of lore in this series: we're never really told if its just the Greek gods that are real in this story, or also those of other cultures? And are they really gods and if not, wtf are they? We know demons exist, and as of more recent games (and especially this reboot) the concept of the demon and the malfested have kind of been merged? Does that include all of the demons in Taki's lore? Who the hell knows, right?
Between that awfully low resolution stone head from Libra of Soul and this, I think it's just their lowest-possible-effort representation of "the gods", that it doesn't mean anything super symbolic about anything. Hephaestus's only "form" we're aware of is the giant stone statue in SoulCalibur III (what the stone head from Libra of Soul was), and that was just a possession. The gods have voices, but not forms. So instead of just having a blank panel, they just grab what's on hand. I don't think there's anything deeper than that.
About demons and malfested, I believe there is still a marked difference between them, same as like the golems and lizardmen. Demons have to be sealed away by ninja magic/scrolls, as they are a natural evil entity that isn't connected to the swords. Malfested are specifically tainted by the swords, and can be purified by divine power of one type or another (Wind, lightning, all that kind of thing!). Golems are artificially created and just... are, can only really be destroyed. Lizardmen are a cult ritual that seems to be a one-way trip and gets you abandoned by the gods, sucks to be you in that case.
Oh but not for Sophie, in whose case one of the powers that be, that moved her around like a chess piece in life, resurrects some echo of her as a host for its desire to impress its will and iron vision of order on the world. And it seems like maybe Cassie met an equally existentially bleak end, because the more I think about it, the more sense it makes that once sealed away with the Astral Chaos, the swords could have reached out to the only thing native to the "regular world", someone with a strong drive and connection to that world, driven by regret over a failure to protect her family.
(And in response to the other part I didn't quote) Yes, it sucks to be the Alexandras, but that's a recurring theme in the series. Sophitia being the form of Elysium, though, that's not quite right. She only assumes that form in order to manipulate Patroklos. I think it stands to reason that the "true form" of Soul Calibur would be that of a water or ice spirit, the opposite of Inferno. It's just that Elysium can apparently take human form if she wants.
I make this distinction, because Elysium spoke to Cassandra at the end of her story in SoulCalibur IV, and while you might could argue that this was only after Cassandra defeated Sophitia in stage 4, this would imply that Cassandra killed Sophitia, and then her soul transcended immediately to be Elysium. That's not true, though, and Cassandra wouldn't have killed Sophitia, for a multitude of reasons, least of all being that in canon, she never reached Sophitia, and was blipped into the Astral Chaos before she could. I'd still submit Cassandra's SoulCalibur IV ending as evidence against Sophitia being the base form of Elysium, however.
We know from Edge master and Patroklos that the Astral Chaos can be used by a very powerful effort of disciplined will to travel back in time. As I recall, Patty's trip was relatively short, but then, maybe in his case it only needed to be. Anyway, nothing about that boy screams endurance, does it? If Cassandra were backed by the power of one or both swords and were to rip herself out of the Astral Chaos and time...that would explain her ultimate fate (as it is known so far). It would also suggest that even that mobile game's plot has been brought into the fold as cannon by this game, to some small extent. But ultimately her goal was to directly intervene on her sister's behalf, or warn her own self. It seems she achieved that at long last (after expending so much of herself that she doesn't even remember her own identity any more), but is she really the only one in charge of these decisions? Or are the swords protecting their own perceived dominion over humankind?
Tying in to the above, though, Cassandra being manipulated by the swords does make a lot of sense, and could add even more tragedy to the family pool. Because we needed more of it. But for all the crap that Patroklos gets, I do feel like his backbone finally came to form by the end of the story, and he would gain endurance as time passes, with the support of Pyrrha by his side. They both need help, not able to do anything on their own, but as a team, they could become quite the force to be reckoned with.
But Edge Master doesn't have the influence of the swords within him, and he seems to be the overseer of the Astral Chaos, though even he fears Night Terror, so he's not all-powerful. Still, he seems to command the time flow without much hassle, though we don't know exactly how he manages. So it's not necessarily a requirement that Cassandra be backed by the power of the swords, it's just that, if Astral Chaos Cassandra's eyes are really red, and that's not just dramatic license, then something has happened to her, where she lost her identity too (though Cassandra not being able to recognize herself was pretty special, not sure how they justify that), it might indicate some level of control. See also Nightmare not knowing that he was Raphael.
...in which case, maybe accepting that "gift" there was the one thing Cassandra could not afford to do if she was going to avert her sister's fate? Man, I take it back...if that ended up being the case and it turns out the family is stuck in an unending loop of misery, that really would be dark... That's basically the ending to 12 Monkeys, lol. ...which is probably evidence enough if any more were needed that this is not where things are headed, because that would be bleak but poignant, and thus have whiffs of a genuine grasp of narrative and plotting. Which rarely finds root in Soulcalibur's soil. :)
Your jadedness about the SoulCalibur narrative aside, with my faith that they can do it right this time, I really do think that it's going to be a tragedy, unless Cassandra is willing to pay the sacrifice to actually change the world, which may come to killing Sophitia outright, as Sophitia is shown to be very stubborn and set in her determination, and by the end of Cassandra's Soul Chronicle, she seems to be, at least outwardly, following in her sister's footsteps in order to become the same type of person, instead of trying to pull Sophitia away from the gods as she herself is.
This isn't the only darkness we have, either. The very real possibility that Raphael becomes Nightmare sooner than expected along with Amy becoming Viola sooner than expected, if we are diverging/rushing things along, depending on how you define it, seeing Amy also lose her identity, Raphael lose his mind, after coming back from Nightmare, not realizing that Viola is Amy (the story I've pitched for "what would have happened" in SoulCalibur V countless times before), that is in itself the same sort of tragedy, but in a different way. Azwel, for all he preaches of "love", really does seem to be about seeding destruction and chaos to all he touches, which leads itself to tragic ends.
Seeing Nightmare claim a life and ruin a soul may also break Siegfried's will, possibly, should things go down a different path, and with him down, Sophitia potentially dead, we're just killing off our heroic support system, and the darkness may run rampant and ruin everyone's lives. Then Ivy's revived human Cervantes has a change of heart and becomes the new hero... right? Or Grøh becomes the anime edgelord protagonist we need, and saves us all through the cringiest way possible.