"The organized worship of the Old Gods was at its highest prior to the First Blight,[...]"
What an interesting coincidence.
Thank you for the extracts, I enjoyed reading them. Is "World of Thedas" worth buying? I read that there are quite a few mistakes.
Except we have very recent and explicit evidence that the opposite was true. I'm thinking this was either one of the mistakes or they just didn't get the memo.
Corypheus and the other Magisters reached out to the Golden City because the Old Gods stopped whispering to them, and worship of the Old Gods was in a sharp decline.
My guess is that as the Imperium expanded, the theocratic elements gave way to just pure power-based magocracy. More Magisters, more apprentices, a focus on etting ahead rather than 'Getting ahead for Papa Dumat'.
I don't have any saves at the start of Here Lies the Abyss.
They're all mid-way through the Fade portion. I couldn't find the Codex entry about the dragon looking like Dumat. I did find the one about the Warden in Fears of the Dreamer interesting in that he seems to be beseeching someone to give him a way to destroy the Blight. Of course, Andrastianism wasn't really at thing at that point. The proto-Maker entity wasn't worshiped. And then you turn in a vial of darkspawn blood to his altar to answer his prayer.
I'm not willing to dismiss the idea that Theodosians correctly identified Dumat/the Dragon that is identified as Dumat. It sounds like the temples, much like the Creator temples, had certain ways of depicting various dragons. That scholars debate for a few years points to it being difficult or possibly just making it up, that scholars come to a grounded conclusion on which Archdemon was which points to there being enough similarities enough that we shouldn't sweep it under the rug. 
I do like this notion that Dumat and the other Old Gods were Dreamers or demons, depicting themselves as dragons, eventually aiming the Magisters at the dragons. (Pillars of the Earth?)
Sister Squish's World of Thedas thing about how some people think that Dumat created the darkspawn and led them against Thedas on his own is quite interesting.
Who thinks that? Old God worshipers? It would be quite the contrast between Andrastianism and Old God worship. 
MAKER: The people have turned their back on me. So I'll return the favor and just ignore them. Screw this. I'm going to make cooler worlds. *floats off* *comes back when he's lonely and wants a girlfriend*
DUMAT: The people have turned their back on me. I'll just kill them off until I can fit the survivors into a temple, and I'll call it good.