Actually the reveal in the Temple of Mythal solved a problem that had always bothered me. If the ancient elves had such a powerful and amazing magical civilisation, how on earth did Tevinter manage to destroy it, even with enslaved dragons? It never made sense to me. However, falling upon the weakened remains makes perfect sense. Basically it would seem that much of the fabric of the empire was destroyed, just as the library was. Then in the confusion that followed, the survivors fought over what was left; may be the slaves rose up in revolt against the remaining nobles, after which they weren't in a fit state to defend themselves.
However, there are also probably two different histories remembered by the Dalish, that have somehow got merged into one. First there was the raising of the Veil and becoming mortal which coincided with greater contact with humans. The Dalish story says that as a result they withdrew from contact. I think that following this several years passed, with the humans from Tevinter and elsewhere periodically coming across elven ruins and plundering what was there, probably killing any guardians that woke to stop them. In Masked Empire Felassan says that the humans copied the runes they saw in the ruins "like children tracing letters". Eventually they probably got some of the combinations right to reproduce some of the magic they had found and so Tevinter grew in power as a result. They were further helped by demons speaking to them from the Fade; the ancient demons did know something of the magic the elves used, probably because they had a kinship to them.
Then some time after the raising of the Veil, when Tevinter had grown in power, WoT2 says that it started to expand its borders eastwards. That would be in the direction of Arlathan Forest. The humans believed the forest was uninhabited but everyone who went in never returned. Likely they had uncovered whatever was left of the central city of Arlathan, guarded by ancient elves, possibly with their descendants still living alongside. Whatever the case, it was this that Tevinter attacked and destroyed, enslaving any survivors. It was this that the elves themselves remembered. As Dorian confirms, the Tevinter themselves liked to boast they had conquered the might of the elves and probably continued to mock their slaves down the years over their conquest, so that is why, thousands of years after (not simply hundreds) the slave descendants actually believed the humans were responsible for the destruction of the original Arlathan, not simply what remained of it.
So whilst Tevinter can be considered absolved of the original destruction and mortality of the elves, since that was down to Fen'Harel, they are still guilty of attacking "the corpse" and enslaving whatever elves remained. I think it is clear that they would never have stood a chance against the full might of the evanuris or even their followers prior to the raising of the Veil.