I don't think we can say for sure either way what's 'to blame' because we know virtually nothing about the eluvians. Given the air of mystery around the eluvians, it's quite possible that they play a vital role in the taint. Especially that one. Andraste's feminine products, ESPECIALLY THAT ONE! Remember that many speculated that what Tamlen described could easily have been The Black City.
Actually, now that I think about it... what if that demon didn't really help Merrill cleanse the eluvian of the taint? Maybe he told her how to break it, severe its link with where ever it goes, and being cut off from the corruption on the other side of its link is what made it stop puking out taint? That would explain why she couldn't fix it and how that demon taught her a spell that's otherwise believed to be impossible (removal of darkspawn taint). Plus it's just the kind of dick move with word games demons love to play.
Duncan was able to sense the darkspawn taint in the mirror. He was able to sense the darkspawn taint in the Dalish Warden if that origin is played. Anders is a Grey Warden. He would be able to sense the taint. He was able to sense the Grey Wardens in the Deep Roads if we save our sibling's life.
Merrill has the Eluvian shard that was tainted for a decade. She was rebuilding an eluvian based on her research into eluvians and what she learned from the shard. She, and everyone in the alienage, were not tainted at all.
After a decade, compared to the thirty seconds for the Dalish Warden, that becomes more than enough proof for me that she succeeded in purifying the taint.
No, they just don't care. There's no reason whatsoever that they had to use Anders and Justice, who could both easily die in Awakening, for the part they played in DA2. They could've made a new character to fit that role, but they disregarded player choices and use them anyway. Very likely ditto for Leliana. I seriously doubt there's any real reason that the seeker in DA2 just had to be the same bard that may have traveled with the Warden. Lots of epilogues were taken back under the guise of "epilogues are just rumors." No they're not. If they were, then some epilogue slides wouldn't have specifically stated that they were rumors. Which they did. It's just plain a retcon, and the writers don't have the guts to admit it. And then of course there's the choices in DA2 which ended up not to be real choices at all.
I just read a new interview with David Gaider today. Don't know if it was today but it was recent.
He and his team of writers (one of them is new, replacing another who is on maternity leave) have to take into account our choices matching what is happening throughout Thedas. Some players did the Dark Ritual. So he said what happens in one play through would be different for a gamer than another who didn't. But because not everyone did, it becomes unrealistic to play the Old God Baby.
They have to consider large events, money costs to make the game, fan reactions to previous games, and then make decisions if all the little quests should follow through into the big picture of later games (DA 3, DA 4.) Large scale things like the Dark Ritual, letting the Architect live or not, will affect the world, but because not everyone did, they have to work with that.
It would take a staggering amount of time and money to take into consideration every single option a player may or may not have done through two games and Awakening when they have no real time frame for how long the series will last.
Gaider said Mass Effect went into from the beginning as a trilogy. The design team knew they had to tie things up or at least make mention of choices that already occurred. Dragon Age does not have the same set up for the games. They don't really know how many games will be required.
But he did say that the option to choose different races may be coming back. Individual origin stories, not likely, but the option to choose the race, a strong possibility. And he did say exploring the Kossith and the Qunari is being considered.