If Marethari had tried, she wouldn't have had enough oomph to do it, and would've needed lyrium or blood magic or working in concert with Merrill. Or some combination of the three.
No, not really. BM is a tool like any other type of magic, so long as it's not abused or used as a crutch, and she's not wrong about Demons. They can be outsmarted if you're careful.
I take it though that you disagree with Solas as well?
As a matter of fact, I do think Solas is wrong.
There isn't a positive way to consort with beings whose very nature is to cause misery. Maybe you really can outsmart lesser demons like Rage, but what would you get out of that? The chances that it intended to fulfill its part of the bargain are slim.
And well, the risk just isn't worth it.
Same goes for human sacrifices. No such thing as "Ohh, the problem is just the WAY they did it!"
Then you could think not all blood mages go that far, but Dorian said it best. "What if you need more? You always need more."
There isn't much room for relativity when "misusing it" is the most common way it's done.