As for the "fall of Arlathan", Solas has already dismissed the idea that the Vints were responsible. What they conquered was probably a pale shadow built on the ruins of the true Arlathan.
I don't necessarily know that we should accept Solas and Abelas' "elvhen" snobbery as the final word on the post-veil elven world. After all, this is the person who would have dismissed modern Thedas as essentially a shadow world filled with non-people.
The post-veil Arlathan no doubt lacked the splendour of Solas' time, but it seems to have endured for centuries and should be considered a civilisation and culture in its own right.
Because a city, no matter how great, would NOT be more famous than an event that remade the entire world. They have 0 information about the Veil not being there. That seems extremely unlikely.
Well, they do remember the creation of the veil as the imprisonment of their gods.
The destruction of the veil was an apocalyptic event. The basis for all their technology was destroyed. Their records were destroyed or lost. They had to try to rebuild their society.
Of course they remember Arlathan better, but that's not surprising. Arlathan fell to Tevinter 1000 years after the destruction of the veil. It might not be the "true Arlathan" to the "true elvhen", but the odds are that
this is the civilisation the Dalish tales remember, through the distorted lens of centuries of illiterate slavery. A civilisation that itself looked back to before the veil as its golden age - and thus buried the truth of Solas' time under a double layer of legend.