Sorry it is taking me so long to catch up! These are amazing Aravasia, good job putting them together! [Solas Approves] 
I may address these individually, as there is a frustrating limit to the amount of quotes in a post!
Mythal as Dumat
Consensus of this thread, I believe, is: agreement for the timing of Dumat's death and Andraste's birth as very suspicious. We generally believe this is being hinted at through the player being introduced to the idea of a corrupted Old God being "cleansed" through being born into an "innocent" body in the form of DA:O's "dark ritual".
There are different interpretations regarding Dumat's identity, however. Most (not all) subscribe to the idea that Tevinter Old Gods are Elven Pantheon members.
It is true that Mythal and Andraste share a penchant for birthing females instead of males. DA often highlights the importance of bloodlines, and "blood" as the manifestation of "will" in general. One interpretation is that Andraste is one of many descendants of Mythal, and this gives her a unique connection to Mythal and her abilities (as seen in Morrigan).
It is possible that Mythal was Dumat, and reborn as Andraste. However, it is equally possible to believe that Mythal, even as Dumat, would not have motivation to convince Corypheus to act as he did. Andraste, like Corypheus, heard whispers (believing it to be the Maker) but the ambitions of the two seem to clash.
madrar theorises that Dumat is instead Falon'Din.
Other theories on the identity of Mythal and Dumat:
There is evidence to suggest that the site of the Temple of Sacred Ashes, where Andraste's ashes were taken to, is also the site which used to be a temple to Mythal. I believe the reasoning involved images depicting Shartan carrying the ashes to the site, linked with the story of Mythal subduing maddened-Andruil at the base of a mountain and history of "barbarian" clans in that area.
(I am failing to find the evidence and will probably come back with it, unless anyone can help out?)
So to address this and Aravasia's theories. 
I completely missed the Sacrifice and Razikale connection.
Previously, the last two dragons being Razikale/Dirthamen and Lusacan/Falon'din just made sense to me, being Mystery and Night, Secrets and 'wings of night'. But Andruil was the one to wander into the Void and come back with some uncomfortable knowledge (before she got mind-whammied), so that could explain part of the mystery. And I like the idea of twin-souled Lusacan and Falon'din hanging out in the same Archdemon.
While I still see a lot of evidence for Shartan=Solas and Andrastythal, I think it's been figured out that the figure carrying the urn is in fact Harvard, and that Shartan is the guy in the background most of the time. They were only working together for a short time, apparently. Shartan officially joined at the Battle of Valerian Plains, one year before Andraste's betrayal. His rebellion had started nine years earlier, coinciding with the Exalted March. So if it was Solas and Mythal collaborating, it was well-played.
Draconis=Tevinter interpretation of Fen'harel. The Kirkwall heraldry did start as a symbol of rebellion and freedom amongst the slaves, many of whom were elven, back in the Emerius days. And a lot of elven graves seem to have it emblazoned on them, and Solas has a scroll stamped with it in his room. But isn't Fenrir also a constellation based off of Fen'harel? There could be multiple, Solas is cool enough for it,
but I'm curious as to how this fits in.
My one problem with Mythal being Dumat is that it involves her aiding the Tevinter Imperium, which doesn't quite add up for me. Assuming it wasn't someone faking a phone call, so to speak. Or it was part of a long plan.
I can see Dirthamen or Falon'din being the betrayer, using the Neromenian Dreamers as minions, much like Dirthie tried with the Alamarri in the days of Tyrdda.
We discussed the emerald waters and the abyss a while back, but you've brought more to the table. It's one of my favorite weird lore things to obsess over.
As for Dirthamen being dead, well, there is that statue of him with a blade through his back gushing blood in the Fade near the end with Flemethyal. 
I'm not sure what to make of Elgie being Zazikel. But none of the other Creators seem to fit.
I like the idea made earlier of the Old Gods actually being the High Priests of each of the Creators, and they're doing their schtick to bust free and rescue their gods. But of course, this doesn't explain the Old God Souls, and Flemeth's interest in preserving the Old God soul makes me think they were either frenemies or the Creators themselves. I like Urthemiel as June, to be honest. The Architect's title suggests he was the High Priest of June, and I associate June's beauty more with architectural grace and giant spikes than a pretty halla-woman.
(Then again, as we all know, the Architect is the prettiest of them all...) But my theory of Ghilan'nain as Andoral (alchemical horrors=slaves is tenuous) doesn't feel strong as I'd like, and it's a good connection. Still letting all of Aravasia's contributions percolate.
I can see things getting complicated with there being some crossovers between all of the god pantheons.
June being a dwarf Titan, Mythal and Fen'harel moonlighting in Ferelden as the Lady of the Skies and Imhar, Elgar'nan in Par Vollen with primitive humans, Ghilan'nain really struggling to explain organized religion to halla...