Merrill - “It's a Keeper's job to remember.”

No one other than Merrill approves of giving her the arulin'holm. Varric and Isabela don't actually disapprove, but the impression is very much that only Hawke and Merrill really think it's a good idea. Poor Merrill!

One of the failures of DA2, I feel, is that it's never really adequately explained WHY the eluvian is important. Oh, we're told about how the elves have lost their culture and this is restoring a small part of that culture, yadda yadda yadda. But it's not clear how the eluvian fits into that culture. Even just a line from Merrill about what she THINKS the eluvian does would have been helpful.

Thankfully, in the wake of DAI, the signficance of the eluvians is a lot more apparent.

The Dalish origin doesn't really teach us anything about the utilization of the Eluvian except that it can potentially kill you.

Merrill does talk about what she thinks the eluvian does though. In the scene where she shows it to you you can ask about it: 

Hawke: "What's an eluvian?" 
Merrill: "Long ago, the elves had a kingdom. An empire that covered Thedas. And every city had an eluvian. The mirrors let them communicate across their empire. But I don't know how, exactly." 

With her talking about how the Dalish lost everything, I don't think she just wants the mirror fixed because it is part of her culture but I think she hopes that it will be able to access others and through that they can learn not just a snippet but everything. Which as it turns out is a correct assumption as we learn in DAI and especially Trespasser, to the point Varric thinks of Merrill when we are learning the truth. 

So, I've been thinking about taking the friendship path with Merrill.

Thing is, I also want to avoid slaughtering her clan and the only way to do it is to promise them Merrill won't harm anyone else with her blood magic. Yes, I know you can max friendship before reaching that point. I'm more worried about whether it would make sense to say that after having supported her goal.

I've considered that, yes. "Maybe Hawke just said what they wanted to hear".

But if Hawke wants to prevent violence, wouldn't it make more sense to defend her innocence than encourage their belief that she is evil?

You're right, but Marethari "fanaticized" them. You saw, what happened with Pol. The word was not enough. The fact, Merrill was full of blood. And Marethari frightened them with Merrill.
If you think, this is the good answer, then try to convince them – but they'll not hear the true word. 

If Hawke in the romance with her – perhaps I would choose that. To protect her in any way, no matter the cost. And I said: PERHAPS. Because of Hawke, who loves Merrill, knows, Merrill wouldn't want to fight against her people. But not in romance? I don't know. Hawke helps Merrill and knows, Merrill didn't kill Maretari. To try to avoid a fight with desperate people isn't that bad decision – and the goal is Merrill's goal, Hawke can interest in, but not like Merrill.

(Not mentioned: this is an idiot answer: probably one of the writers suggest, like the friendship is mad, the rivalry's the best way ever – because these character's flawed, Hawke must fix them, and the friendship with them just "enables" their wrong views... – but according to me, this is wrong.)

Again: it depends. Hawke says, if I remember, s/he will take Merrill away. Not blame her or something. But this is one of the stupidest answers in the game. As I said: it's about: the rivalry is The Right Way(TM), because Hawke must fix his/her companions as a broken car...

This game is the most beautiful piece I ever played but has many flaws... an unpolished diamond – whit some suggested bullshit. Still fascinating.

You can avoid killing them all if you run to the zone exit farthest away from the camp and leave the zone in the short period between waves when you are out of combat. Don't remenber if the game still counts it as if you killed them all anyway tough.

I suppose, yes. She said often. And that scene in the Fade just would strengthen her. She called Hawke for help. If she would want to accept Audacity's offer, she would do it alone. But she called Hawke to prevent it. And Anders frightened her enough when he spoke about his personal experience grossly, I suppose.

I didn't read Merrill's short story, and I'm working now, but later I will. So: perhaps, I'm wrong, but I think, Merrill can handle the problem well.

Hawke can't know the outcome, but s/he can try to guess, right? "Mm... what can I say to avoid pissing off these elves even further?"

Also, congratulations for your modhood.