How I see it is the Taint itself is older than the Darkspawn. But the Taint of Red Lyrium is different than the Darkspawn taint. They infect and effect people differently.
Consuming Red Lyrium will cause the lyrium to grow within you, but it doesn't turn you into a mindless slave.
Darkspawn taint will turn you into a mindless slave or change you into a broodmother (female).
So my theory is the taint is older than the darkspawn. Therefore the red lyrium taint has different properties and will do different things. When the magisters entered the Fade they brought the taint back into Thedas, but it was changed. Evolved???
Think of it like the real world flu. Bird flu and swine flu are both the flu, but they both behave in different ways and are transmitted in different ways. Still the flu, but different because they come from different sources.
But this of course just begs the question, where did the taint come from and why is it here?
This is the same basic theory I took away from DAI as well.
In regard to your final question, I recall a banter between Cole and Solas. Like many of the banters between the two of them, it begins and ends very abruptly.
Cole: *gasp* A war in the Fade, waged with human hatred.
Solas: It would be a terrible thing.
Cole: They shouldn't have put it in the girl. It hurt her!
Now, it could have absolutely nothing to do with the taint of the blight and the Darkspawn. But hte idea got me ot thinking. If the theory of the taint predating the Darkspawn holds any water, as suggested by the possibility that Red Lyrium might have existed long before the First Blight began, then it could be older than the Imperium.
The empire of Elvhenan is older than the Imperium, and information from DAI paints a grim picture of what that empire might have been like. The kind of place where a "god" might create a weapon so incredibly dangerous, even to the creator, that it gets buried in the deepest darkest hole he or she can find. Or jettisoned into the spirit world.
Just food for thought. Guesswork. 