To be honest we don't know fully how the Dalish may have viewed their gods because we can only go on what we have been given in the past and clearly things were left out even from their own more recent history. How would a Dalish not know something about the history of Ameridan when his own clan had been claiming ever since that he was one of them? How would the Dalish know nothing of what happened at Red Crossing when someone had to bury the Emerald Knights in the tomb and seal that history there? Then Morrigan springs that story on us about Dalish hunting humans by invoking Andruil. We can repudiate that as a Dalish but if it is true, then surely we would have at least heard rumours about these antics of other clans, which would bring us all into danger and the clan would have been censured at the Arlathaven.
Even so, the story of Elgar'nan does depict a more ruthless, violent sort of person and he is after all called the god of vengeance, so the Dalish realise he isn't a particularly benign god. It may even be that the stories about the Creators have some degree of truth in them if you think these hark back to the early days, long before the rise of their empire. We still do not know exactly what the Evanuris were originally. Flemeth called Mythal the wisp of an ancient being not the wisp of an ancient elf. The strange depiction of what is likely meant to be Mythal in the Deep Roads in World of Thedas p,.38-39 does not look anything like an elf, neither do any of the statues we see. DG originally said back in 2009 that the Dalish did not believe the Creators created the world but just what came after. In other words they were responsible for building up their civilisation from nothing. (World of Thedas contradicts this by saying that the Dalish believe Mythal created the world). Still if you interpret this as Mythal and Elgar'nan creating the world of the elves, then the belief is consistent with that earlier assertion. In that case, everything the Dalish says the Creators originally did for them may have been true. They did teach them all the skills they needed to survive, about building fires and healing with herbs, about crafting bows and swords, about the value of family and community. That would explain why they think of them as the Creators and not the Evanuris because the former name belonged to an earlier era when the world was younger and everyone was more innocent. Even Solas admits that the rot set in after the war in which they were generals. Then after they were taken away by the Veil and the world of the elves crumbled about them, then their human neighbours preyed on them and enslaved them, it was only natural that they forgot the bad aspects of the Evanuris but recalled rather only the goods things that the Creators had done for them and what a beautiful world they had created for them. Solas is really no different. Even though he knows how bad the Evanuris were, he is still nostalgic for the world they created and wants to restore it. I dare say he even has a rose tinted view of Mythal. "She was the best of them." No, she was probably exactly the same as them but she just had a different agenda from the others.
The main argument my Lavellan uses against supporting Fen'Harel is not whether or not he believes his assertions about the Creators but the fact is they just don't need them. They have managed to survive down to the current time without the aid of any gods and they may not have an actual homeland but as nomads the whole world is their homeland and now Solas wants to even take this from them. Even the humans couldn't do that.