*gets the crazy eyes and hisses "wikiiiii"*
I'm not on board, but would take a few pages to explain why. Interested in taking these things apart, or just want to keep them as they are?
They are hypotheses, just as yours are. They have flaws, but so do yours. Neither is likely to be canon.
I'm just poking around with ideas I've had; there's actually pretty good evidence in the codex entries and dialogue (the sinner codex mentions gods and their chosen being allowed the divine winged form, Abelas tells you that people like the Venatori murdered Mythal, if you side with the templars and fight the Venatori, and we know that they took down Mythal instead of Elgar'nan). BUT, the codex entries have proven to be unreliable, and character dialogue relies on that given character's perspective, so take that as you will.
As for the Anchor, or Mythal being head of the pantheon, there's no real evidence one way or another, but the comment many pages back that there is more than one focus suggests to me that there are more than one. But, the Anchor was once part of Solas's orb, and that one's just wild theory. There's no evidence either for or against it, except that an anchor holds something in place. Does the Inquisitor need to be anchored into place? I doubt it. What else needs to be anchored into place? Maybe something that might have to do with magic tied to the Fade? ... Hm, the Veil? So it's a guess. Probably a wrong one, but I'm not throwing theories out there to be right. If I'm right I'll be highly entertained. If I'm wrong, I'll be like: Ooooh I was wrong, it wasn't for anchoring the Veil, it was for X reason! (Bearing in mind that I'm probably wrong because Corypheus did some kind of ritual to get the Anchor and it took him years--so the orb's original purpose could have bee something entirely different. Perhaps the Veil was torn in the past as well, shortly after its creation, and the orb was made to help, or perhaps it has nothing at all to do with the Veil. But, if the Anchor DID exist before, and Solas DID wield it, that still doesn't necessarily mean he ever walked physically in the Fade, which I am sure is another argument against it. It's likely he was unaware that he would be able to do such a thing.)
Back to my concept about Mythal and her murderers:
Was talking in chat just now about that idea about the Old Gods NOT being the Creators, or keys, but the chosen of the Old Gods, and it led to the discussion of what Venatori-esque group killed Mythal. It could have been the "sinner" from that one codex, sometime after he was punished. The Creators themselves may have been involved, since the sinner said Dirthamen ordered him to do what he did. More accurately, the "sinner" felt wronged, especially if he'd been telling the truth, so he may have gathered followers after being corrupted/punished/etc. and set forth to become a god himself. Or maybe that was his goal all along: to become a god. It seriously looks as if the ancient gods were playing the Orlesian Game, so, who can say?