If cross breeding wasn't the reason magic got into the human race, I'm pretty sure that Dreamers did have elf blood and the reason they are becoming less common is the dilution of the elf blood in the human families and the dilution of the ancient elf in the modern elves.
Mihris (in Masked Empire) mentions how the Dalish believe that the ancients in Uthenera could see the whole world, could tell their followers where their enemies would be and in what numbers and, where the Veil was thin, even send spirits to do their bidding and assist them. Felassan mocks her ideas but he is so derisive of them, that leads me to suspect that it actually was true. If so, that would account for why the Tevinter Magisters wanted to kill them rather than consult with them. Even if they had asked, I doubt the ancient elves would have been forthcoming on the issue. According to WoT2 when the city was discovered in the Forest of Arlathan, the Tevinter initially sent emissaries to make peaceful contact with the elves but when none of them returned alive and people kept disappearing or turning up dead, they finally decided to take action. I think it is pretty clear that the elves did not want to share information.
As for the ancient ones waking up and telling modern elves how the Veil came to be created; well they don't seem particularly friendly to intruders, so any post-Veil elves that weren't actually their followers would simply be killed. Besides which, we talked with Abelas and he never said anything about the Veil being raised. The only reference you have is in the ancient text that you can only read if you drink from the Well. In it he says "We are trapped (clearly he is referring to be trapped this side of the Veil). The ones born here do not understand the keenness of what we have lost or why so many of their elders weep as they enter uthenera. (It would seem that those elves born post Veil find it difficult to understand what exactly happened; however the knowledge was passed on down to the Dalish that their ancestors were once immortal and magical). Even the memories in the Library seem confused as to what exactly occurred. One group ask "What has Fen'Harel done?" Another group keep asking "Where are the gods?" Again these memories were passed down to the Dalish; Fen'Harel did something that took their gods from them but it is entirely possible that Solas is the only one who actually knows the mechanics of what was done, in other words the creation of the Veil.