And then Marethari confirmed beyond doubt that Merrill was better than her, by getting possessed.
Also, it wasn't everyone. It was Anders, Fenris, Marethari and her goons.
Admittedly, the people who disagreed with Merrill weren't as informed as she was (and Marethari's dialogue implies she didn't research it because she felt the technology should remain buried), but I think cultural differences also play a role. Merrill's reference to the denizens of the Beyond as spirits isn't elaborated on until the religious arguments between Merrill and Anders (and confirmed by WoT), where it's clear the People don't make distinctions between "good" and "bad" spirits (i.e. the First Children of the Maker, and the ones who turned their backs on the Maker in jealousy over humanity), but instead view all spirits as dangerous. To the Andrastians, demons are pouring out from the Fade; to a Dalish Inquisitor, spirits are coming from the Beyond.
Unless the unique cultural perspectives of the racial backgrounds are diluted because of the voiced protagonist (and the elven perspective was, by the author's own admission, removed for the Dalish in "The Masked Empire" to avoid confusion for the readers), I wouldn't be surprised if this was also an issue with a Vashoth Inquisitor (depending on how differently the Qunari view the "Land of the Dead" in comparison to Andrastians) or especially an elven Inquisitor, given the relatively high Andrastian composition of the Inquisition (including all three advisers, like Josephine).
Of course, that might not generally be the case. Given Zevran's reference to the dangerous Dalish of Antiva City (which indicates the People have a presence in Antiva), Josephine may have some experience with the Dalish, which isn't necessarily the case with the other advisers (as The Warden was the one who primarily dealt with the members of Zathrian's Clan, while Cullen seemed to be handling administrative duties at the Gallows, rather than pursuing Dalish mages).
Given the severe lack of information about the Vashoth, I remain uncertain how that will be handled (I'm hoping an atheist perspective is available since the person isn't a follower of the Qun, similar to the one that was available for The Warden). It might involve the Vashoth protagonist overcoming the perception that he or she is Qunari (most likely among the Andrastian companions and advisers) Given the proximity of Antiva to Rivain (where there are some Qunari converts), I suppose it isn't impossible for there to be some Vashoth (or Tal-Vashoth, at least) in Antiva.