Guess I might as well post my own opinions. I trust Merrill, personally. She made her own eluvian from a single shard and I fully support her trying to build one, has absolute moral and ethical fiber that I know means that she would never use anyone else's blood or sacrifice anyone (for seven years the only blood she ever even considered using was her own), the eluvian she built was no danger and I fail to see how it could have possibly led to the demon's prison and in fact I'm quite sure the thing was after the Keeper (and it might have been a harmless wisdom spirit before the Keeper corrupted it by letting it possess her), she is actually pretty smart about demons and spirits and their danger (Merrill to Anders: "There's no such thing as a good spirit. There never was. All spirits are dangerous. I'm sorry that you didn't"), and I don't think blood magic is bad if you're responsible about it (no non-consensual blood sacrificing and no mind control and not used as a crutch... Merrill had trouble with the last part).
That is no where close to inevitable. Morrigan also knows and uses blood magic at times, yet she's never made a blood sacrifice of anyone. Most mages are too weak for it, but some like Merrill and Morrigan absolutely can be users of blood magic without jumping off the slippery slope.
I don't think Merrill used it as a crutch. She learned blood magic as a last resort.
She talks about how she brought an eluvian shard from Ferelden and the Brecilian forest and asked Merethari to help her cleanse it, and Merethari refused. Merill will tell Hawke she could have done the same thing if she had plenty of lyrium on hand to amplify her magic but she didn't, as the Chantry controls the lyrium trade (she doesn't actually talk about the Chantry, but I infer that is the reasons she doesn't have it,) and she went to Audacity to learn blood magic as the work-around.
David Gaider said a few years back that Merrill extrapolated knowledge from the shard and what she could gather on elven lore to build a new eluvian, so she wasn't communing with the demon for information. She went to him a total of three times. The first time was in the short story written by Jennifer Hepler that takes place before the game where Merethari and Merrill investigate the presence that is affecting them in their dreams (and Merethari was waking up in tears and was far more affected than Merrill,) and Merethari declares the demon harmless. The second time was when Merrill went and learned blood magic, and the final time was when Merrill and Hawke went up to find out what is needed to activate the eluvian only to find aboination-Merethari.
I don't blame her for what happens to her clan and Merethari, because doing so takes away the fact that the other members of the clan are thinking adults who are responsible for their own actions, and Merethari is the one who acted rashly and did more to put her clan in danger than Merrill ever did.
And I think that Inquisition and the Masked Empire together prove that Merrill was 100% right about the eluvians.