I keep trying to point out that a lot of the arguments for and against Merrill are based on hindsight. I try and view it from Hawke's perspective at the time. Yes, it is quite possible that Hawke was being hypocritical going on a dangerous mission to the Deep Roads and then not helping Merrill. However, it should also be noted that her request comes after that nearly fatal experience, so it could be that Hawke is now rather wiser about jumping into danger. The argument that Hawke had alternatives to going to the Deep Roads is also a moot one since the writers do not give us that option, nor do they allow us to say categorically that we do not want to go to Kirkwall in the first place. What they do allow is for Hawke to make an informed decision about whether or not to help Merrill, based on the evidence available to them. From Hawke's perspective, there is no guarantee that mending the eluvian is going to help the elves in the slightest in the present but it might well get those in the alienage killed if it doesn't work out as planned. If I had been playing Hawke who was a Dalish or even a city elf, I might have had a different perspective on it, but I was playing a human, who was trying to improve their lot in the world and, if a mage, trying to avoid getting arrested and locked up in the Gallows. To my mind, common sense dictated that they would be very wary about helping Merrill and more inclined to give credence to the advice of a clan elder. They might even think, "what would my father have done?" I don't think Malcolm Hawke would have risked his family for such a venture.
As for claiming that Merrill is so much more knowledgeable than Marethari about the eluvian, I thought the game made it clear that the knowledge that helped her cleanse and mend the eluvian came from Audacity. This is why Hawke is so dubious about helping. I'm sure if Merrill had produced a translation of the elven, giving her the details about eluvians and their construction, that would have been a different matter. Not only that but the companions are not encouraging either. Anders, the promoter of mage freedom, is constantly saying what she is doing in consulting a demon is a bad idea. Considering he is actually an abomination with a spirit inside him, that is pretty hypocritical, but nevertheless you could call it expert advice on the matter. You say Isabella, Varric and even Aveline might have been in favour of helping but I doubt it. Isabella is anything but stupid and knows the dangers of demons, Varric is wary about strange magic and Aveline would give deference to Dalish authority (in the form of Marethari) on such matters. Just because you like someone, doesn't mean you necessarily give into everything they demand. On the contrary, you are more likely to refuse because you care about them. My Hawkes who refused to help, or kept the knife, did so not because they didn't sympathise with Merrill or were unfriendly towards her, but because they were genuinely worried about what she was doing and how it could impact on her.