Honestly I doubt the eluvian being intact would be nearly as negative as the lyrium idol. The eluvians are essentially just a portal network. I imagine if it were kept intact it would have "come online" at the same time as the rest of the eluvian network did in The Masked Empire.
I didn't read the Rivalry scene as literally. Smashing the Eluvian, as I saw it anyway, wasn't about smashing the Eluvian. It was about Merrill coming to terms with the fact that her naïveté is quite literally destroying her life and it'll only be a matter of time before it claims her too. It's not the mirror that is the problem, it's Merrill's obsession with getting it working that is the problem. Look at the price she pays for it: Marethari's life, potentially the life of her clan, and the only livelihood that she has ever known.
If you compare the dialogue, the Friendship version blames Marethari and the rest of the clan for not trusting her judgment. The Rivalry version blames herself and her unwavering passion for being the source of her problem. That right there is why I choose to smash the Eluvian every single time. It's not the mirror that is dangerous, at least not as far as I can tell, it's the fact that Merrill doesn't seem to learn anything at all from the dangerous lifestyle that she leads if she doesn't smash it. I love Merrill, she's my favorite character in Dragon Age II, but let's be honest here: she is the poster child for why the Templars favor treating mages the way they do.