@Nyte -- I didn't have enough time to write everything I wanted to say out as a reply, so I'll do it now. I'm just thoroughly convinced that you hate SoulCalibur V so much, that you're pulling every mental gymnastic possible there is to let yourself believe that we'll never get back to that time, when there's plenty of events in SoulCalibur VI that foreshadow things that happen in SoulCalibur V, as well as SoulCalibur IV, SoulCalibur III, and SoulCalibur II.
You opened strong, and I agree with your opening sentiments, that SoulCalibur VI is indeed consolidating the events that were haphazard at best and confusing at worst, and making one cohesive narrative. That much we agree on, and I feel like everyone can agree with this. The part where you jump ship, in my opinion at least, is where you're saying that new elements introduced automatically make it a new timeline, and that's not the case. You can use the word "retcon", if you like, but that doesn't mean it's really what's going on. It's not a retcon in the way that it's writing new history that didn't exist before, because almost every event that is in this game is also somewhere else, be it shown in a scene/ending, written in a bio, or worst case, written in an external material such as an artbook. And yes, this includes SoulCalibur V as well as the other games. They're foreshadowing events from ALL of the games. So let me address each of your points, and then I'll add a few of my own.
I don't believe that we're seeing eye to eye on the Siegfried event. This event is not showing that Siegfried defeated Soul Edge and became the wielder of Soul Calibur, I don't see where that's the case, in any respect. What this scene is doing is establishing the existence of Soul Calibur (Evil), and by extension, Elysium, the part of Soul Calibur that lives inside of Soul Edge. As Zasalamel puts it elsewhere, the two swords are made of the same steel, they are, in fact, one in the same, just each with its own "soul" that drives its values, which was also emphasized and was a core theme of SoulCalibur V. They're not so much "evil" and "good", but "chaos" and "order", the two sword spirits. Siegfried confronts the sword and establishes the identity, but still ultimately fails, and he's still going to fall into becoming Nightmare again for the events of SoulCalibur II. This whole Astral Chaos thing is all in his head, he fights Nightmare, he fights Inferno, he does defeat them, but he still acknowledges he lacks the strength to be rid of them for good. He doesn't take Soul Calibur, either, it's just foreshadowing that one day he will, once he does overcome his curse.
I will concede that yes, Geki is the one who takes in Gel-o-Fury this time instead of Toki, so that is a new thing that happens, but the outcome is still the same. Gel-o-Fury still manages to escape in the aftermath, and New Legends of Project Soul also indicated that Toki was still alive, so the end result of the matter is still the same. Gel-o-Fury escapes, will eventually be absorbed by Soul Edge, and Toki is still out there alive, somewhere, and Taki doesn't know where. I would see this as an event of making things cleaner more than it is changing history. All the key pieces are still in play, same ending, different process to get there.
Raphael's story before SoulCalibur II, we didn't see in-game before, just as we didn't see Tira's, and by extension, the Bird of Passage, in-game before either. Linking the two subplots together works as a nod to the conflict between Raphael and Tira in the opening of SoulCalibur III, something that wasn't touched upon in SoulCalibur III. Now it kind of makes sense why they would be fighting each other, should Raphael find out that she's associated with the Bird of Passage due to her history. All we did know is that Raphael had to flee France to get away from gnarling nobles, which he did, that Amy would save him, which she did, and that eventually, they would return, which they did. We got proper details and context, but nothing was changed, in that everything that happened before still happened, we just got to see it.
You're very much stuck on this whole Jolly/Gloomy Tira thing being a divergence, when it's not. Tira is confirmed unstable back when she was nine years old and active in the Bird of Passage in New Legends of Project Soul. While it wasn't a gameplay mechanic, she clearly had both Jolly and Gloomy in SoulCalibur III, so the idea that it wasn't manifest before then is just malarkey. New Legends of Project Soul also already confirmed that the Evil Seed drove everyone mad and broke the leadership structure of Bird of Passage, not killing them. Tira was nine years old in 1983, which is where this timeline starts. The only thing that may have been changed is her age, but that age can also be her age at the end of the timeline, 1590, which is when SoulCalibur II through SoulCalibur IV take place, so not even that. All this is, again, is us seeing her story play out in-game for the first time.
Yes, King Arthur and the Aval Organization are new. But they always existed, even if we didn't know about them. It's stated over and over again by members of the Aval Organization and it's written in the texts about how the Aval Organization, and by extension, King Arthur, are well-kept secrets that no one in the general public knows about. They're so secretive that if their members go rogue, they kill them immediately so that they can keep their cover. No one in the world really knows about Soul Calibur except for the few who have been in its presence, as the Aval Organization keeps it a close secret within their ranks. This was preceded by the keepers of the spirit sword, which Zasalamel was a member of, but it's all Aval Organization now. By contrast, Soul Edge is practically common knowledge, and there are even people who believe that it is the savior, because they don't know that the sword Soul Calibur even exists. But as stated before, they really are one in the same, so it's kind of ironic that they're kind of not wrong.
I've already really covered Zasalamel above, but it follows that Zasalamel, an immortal who has existed since the ancient times, back when Algol was originally a thing, when he created Soul Calibur from Soul Edge, so he knows the truth, and obviously, he keeps that to himself except for the select few who might be able to help him achieve his goals. This isn't really "new", it's just that Zasalamel's motivations weren't spelled out before, but they make sense in context with his actions, and this game even sets up his future in his final Soul Chronicle chapter that you unlock in Libra of Soul.
I'm almost getting the impression that you haven't played/finished Libra of Soul, too. Libra of Soul is absolutely canon, and there's nothing inside of it that contradicts anything else either. Much like the Aval Organization was a kept secret, the Conduit (your character) was also a pivotal role in keeping things in check, assisting Grøh and the others in putting down Azwel for defecting and threatening the well-being of the world. Without the Conduit, Azwel's plan would have succeeded, and that really would have established a new timeline. In that Azwel failed, history continues as it was, but as Raphael's final Soul Chronicle chapter that you unlock in Libra of Soul confirms, Azwel actually does survive, and it is he who aids Raphael in pursuing Soul Edge, giving him the motivation to go after it in SoulCalibur II, when it wasn't really clear why he was the one to stab Soul Edge in the eye originally, why him and not the heroic trio of Kilik, Xianghua, and Maxi.
Raphael's story is definitely helping to set up SoulCalibur V, as it confirms that he is, in fact, Nightmare from SoulCalibur V, as if we didn't already basically know that anyway. We're getting foreshadowing that this happens, because he's taken up residency with Graf Dumas, and, his updated bio in the museum, after getting his final chapter, actually states that Raphael poisons Dumas, and that's how he will assume Dumas's identity. He's already in his estate, he's killed the real one, and no one knows he's the fake one, so that's how that'll go moving forward. It's gonna take going after Soul Edge, failing, surviving (thanks to the medicine that Azwel gives him, which, before you say anything, doesn't decanonize the idea that Amy is the one who helps him get better, because he will definitely still be sick, and Amy may have to be the one to administer the medicine), and going mildly insane that Amy fell to corruption too, before finally falling and becoming the new Nightmare after the events of SoulCalibur IV.
And for my last point, since we already basically know that Amy will be Viola in the future, and Raphael Nightmare is still going to happen, the only missing piece of the puzzle, then, is Z.W.E.I. -- or is he? Grøh's fate is uncertain, per the ending of Libra of Soul, he can't return to the Aval Organization, so he will have to pursue his own path as a lone wolf... hmm... yeah, I don't think I have to say it, I think you can figure out where I'm going with this. I've been saying it since we first even knew of Grøh's existence, and I'm even more convinced now than I've ever been before. I can't wait for SoulCalibur VII to continue the threads of this and get us back to SoulCalibur V, probably in SoulCalibur VIII, and we'll find out the truth. The difference, though, is that I don't think we'll timeskip or miss half the cast. We'll get the best of both world, keep the old, establish their presence, get the new, establish their presence too, and move forward amicably and go from there.
Libra of Soul has to be canon, because if it's not, then Raphael doesn't get the information from Azwel's research, nor does he meet Azwel, it's pretty important, the events that Grøh and Azwel do in Libra of Soul, and that will tend to the future. There isn't any other way to justify things. Soul Chronicle was just a prologue for Aval Organization, their full story was told in Libra of Soul, and showing their involvement completely. As far as multiple choices go, well, I'm about 2/3 through my second playthrough of Libra of Soul, and making choices the other way doesn't make hardly any difference, so far, I'm not even expecting the ending to be much different, but I'll find that out tonight.