I'm glad to see so many posts here! I love discussing DA lore, so thanks for participating!
Edit: I don't know why the quoted names don't show up! They're there when I edit the post, but not visible in the thread. 
Edit 2: Aaaand they're back. This is confusing. :/
Andraste as the first Old God Baby is an interesting theory, especially since there is not much lore on the First Blight. You don't know the name of the Warden that ended it for example, so it is possible that someone could have struck down the archdemon and lived.
Yes, since we've so far actually learnt very little about the inner circle of the Grey Wardens, and most important: we've learnt that they don't know it all, I'm sure that there's a lot of potentially volatile secrets that they're keeping from outsiders.
I think it more likely that the Tevinter idea is correct. Andraste was simply a very powerful mage and probably a dreamer. She was contacted by a powerful Fade spirit that called itself the Maker. It is possible that it even possessed her for a time. We are told this is what happens with Rivanni wise women and they are highly respected in their culture and leaders of their people. Even the Qunari thought it wise to allow this to continue rather than try a forcibly remove them. So they are probably a throw back to a time when wise women were a common feature of the barbarian clans. When non demon Fade spirits react with the mortal world, then then to be very powerful and when they manifest themselves, it can be awe inspiring. If Andraste occasionally used to glow like Anders did when Justice rose to the surface, then I'm sure that would have been a pretty powerful incentive for people to listen to her. Since she was advocating throwing over the Tevinter Imperium, that alone would have made her popular. When Maferath's armies started being successful, it would have only added to the charisma of her appeal.
This theory is also interesting, and one I must have overlooked. I'm quite convinced that the Maker is not a God in the way most people would think of one, but an immortal being of some kind. If Andraste had come across one in the Fade, unknowingly, then that would also make sense. Good point about the parallell to Anders and Rivaini seers possessed by spirits! I do hope that we get to visit Rivain in an upcoming game to allow us to find out more about the practice.
I've said this 100 times. Andraste is the Dragon of Silence. Another piece of info that might contribute to this theory is that The Sentinel set allegedly belonged to the proto-Warden that struck the killing blow. Why would this armor set find itself in Ferelden and not Tevinter? I speculated that this Original Warden was Andraste's father and journeyed South to Ferelden with Brona (who was a Warden? Bloodmage?) as far away from the Blighted lands as possible so as to not taint the child.
Years later Archon Hessarian conveniently converted to the Andrastian religion as Andraste was being burnt at the stake. He used his Sword to mercifully end her suffering when at her death he realized that she was the Dragon of Silence....
Two excellent points! I had forgotten about the Sentinel armour set. One of my theories above is that the soul would have been reborn no matter what, but since the war had gone on for 190 years, I'm sure there was at least some Wardens traveling with their families, as well as camp followers and Wardens hooking up with other Wardens. So the possibility that Brona (if she really was Andraste's mother) travelled with the army is at least not non-existent. Though if you follow the messiah parallell in the myth of Andraste, immaculate conception is also a possible reason I guess.
I agree. Why can't Andraste just be Andraste?
She absolutely can! But it's fun to speculate and try to connect the little pieces of information that Bioware seems to have spread out through the games and spin-off products. 
If you take oghren with you he tells you how that mountain contains MASSIVE amounts of lyrium. Also we have no idea what magical protections or effects may have been put on them when they were placed there.
I've probably never brought Oghren there, and thus missed this. Perhaps it was used to preserve her ashes? Lyrium wouldn't act as a cure by itself I think, though possibly it may have been used as a catalyst.
Perhaps I'm remembering it wrong, but wasn't the story about several warriors killing Dumat a retelling of the first time he was cut down before it was known that the archdemons could respawn? I believe the story of his final death hasn't been told (yet) in the lore.
The darkspawn blood idea I believe came about by accident. A barbarian warrior that came from a tribe who had a tradition of imbibing the blood of fallen enemies did so with darkspawn blood that he had mixed with a lyrium potion. Quite unintentionally he became the first Grey Warden, realized it enabled him to sense darkspawn, and passed the practice on to other people combating the Blight.
I'm quite convinced that we've not been told the whole truth. A lot of facts told and texts that appear in DAO seems to be purposefully misleading. We've already seen some theories debunked in DAII and it's DLC (like how the Magisters weren't responsible for bringing the taint to the Golden City), so I'm sure there are other, even greater revelations yet to come.
Your theory about how the Grey Wardens found out about the special properties of the tainted Darkspawn blood actually sounds quite plausible. Perhaps someone who was a reaver? Or did Theirin invent reaving?
Another thing that's interesting is that Genetivi tells the PC that practices/rites of Andraste's Cult predate the Orlesian Chantry. So ironically, the Reaver's version of the Chant and Andrastian religion is closer to the practices of Andraste's day.
Yes, the cult does seem more like a primitive religious practice, while The Chantry lore appears like a white-washed version of what actually happened.
Let's keep speculating! And if we run out of ideas, I'll chose a new topic for us.