But at the same time, if Merethari hadn't refused to help Merrill to purge the mirror then Merrill wouldn't have feelt forced to do it herself and turn to blood magic.
Unless Merethari was going to raid Kirkwall and try and steal Templar lyrium supplies, Merethari's assistance wouldn't have changed the need for blood magic. It might have legitimized Merrill's attempts... but, at the same time, could well have delegitimized Merethari herself. Condoning taboo for Keeper's Favorite would hardly make it no longer a taboo.
Now, if Merethari and the Clan hadn't been oppossed to Merrill's efforts, would the events have occured as they did? Certainly not- hence why TK doesn't lay all the blame on Merrill. But the same can be said of Merrill, who has a lot less legitimacy in her stand of refusing to backdown than the Tribe had in refusing to support and enable her actions.
Most Dalish feel strongly about getting back what they have lost, it's the most important thing to them, but Merethari are different and want them to forget the past and move on. Most Dalish would probably have agreed with Merrill and helped her because just like her they too want to find out what they have lost and get it back.
Except that 'most Dalish', which is to say every Dalish in the game who wasn't named Merrill, didn't agree with her or help. Not just Merethari, but the hunters, the craftsman, the recent convert, and every other Dalish in the game who offered a view on the subject. In so much that there was ever a 'typical' Dalish stance to be presented, Merrill the Pariah was not it- the 'typical' stance would have been the sentiment shared by the more numerous and 'normal' tribe. And the view o f the normal Dalish was that Merrill, and her work, were wrong.
The Dalish have a cultural reclamation complex, but they also are superstitious and defer to their leadership on places to avoid, even if those places and things might have some scrap of cultural artifact. There are hardliners and other clans (or rather, other keepers) who might agree- but it's extremely unlikely they would be presented as representative of 'most' Dalish when the writer intent was clearly that Merrill was outside the norm.