Well, Clan Virnehn never "violates the tenants of the Dalish, including the prohibition against using magic that involves spirits", as far as we know. What WoT says is that Dalish "do not use any magic involving spirits, as they believe all spirits are dangerous". It doesn't say it's a prohibition; it could be, but it could also be just a matter of tradition. Isn't Merrill Dalish after all?
I'd consider Merrill an unorthodox Dalish, but even she was ostracized originally for speaking to a spirit and using a school of magic that involved a spirit. I also never said that Clan Virnehn wasn't Dalish, although I do consider it unfortunate, given the lack of depth to the characters, and their existence solely solely as a plot device for the Eluvians.
However, Clan Virnehn is as representative of the entirety of the Dalish as Alrik and his accomplices are of all the Templar Order. There's no reason to consider an atypical clan as representative of the Dalish as a whole, particularly when we have numerous examples that disprove this notion - including ones provided by the developers in addressing how varied the Dalish clans can be. No group is perfect, and the Dalish are no exception. Even one-dimensional cutboard cutouts like Clan Virnehn.
Their view about the City Elves is extreme, true, but it could also be a natural deviation of typical Dalish condescension that is a part of their culture. The Dalish never hide that they consider their way the "true" elven way, in what Fenris calls "a smug sense of superiority", and there are enough examples of that in the past games, including the Dalish entry about the City Elves and Paivel flat out telling a Dalish Warden that "we call them flat-ears, for they differ little from their shemlen masters".
The fact that some Dalish look down on the city elves for living under human rule doesn't change that one of their primary historians, and their hahren, talk about the city elves being part of an independent elven kingdom. Even Paivel, who you cited, talks about how the Dalish can learn from the city elves, despite his feelings that the Andrastian elves are culturally human. We have Dalish accepting elves who lived under human rule, and even rescuing a young boy who was left to die by the templars, while Clan Virnehn expresses an opposing view on the city elves that doesn't seem to be shared by the clans we have encountered so far; I continue not to see Clan Virnehn as representative of the multitude of Dalish clans out there.
However, until we witness an Arlathvhen, we won't really know if Felassan was telling the truth or not, if Clan Virnehn's views are extreme and in the minority, or if they are the general rule (an absolute rule is impossible, in one way or another).
How are they the general rule when they explicitly contradict what one of their primary historians and the hahren say about welcoming the willing city elves into the Dalish and how WoT reads that the Dalish don't use magic that involves spirits? Felassan's repeated disdain for the Dalish isn't sufficient enough to consider me that all the evidence to the contrary is wrong.