I think his People have been around us from the fist game. We just didn't realize it yet.
We now know that the elves’ immortality was connected to the Fade somehow. And the price they had to pay for it was being bound to it for eternity.
I think the elven Uthenera(endless dream) was in fact the Fade pre Veil. At first they needed to perform a ritual to stay there for long periods of time, but after a time they could do it at will.
From the information we have so far, the elven version of the Fade had no demons what so ever, people actually preferring it to the real world when they got to bored with immortality. There were some spirits there, because when they returned from their sleep they seem to have learned a lot of new things. Or maybe they learned those things from other elves they met in the Fade, or their gods that were ruling them from the Golden City.
So when Fen’harel raised the Veil, he couldn’t warn the People about it because the gods could find out and return to their bodies, thus escaping the trap.
Since the gods themselves were unable to return to the waking world, I’m sure the normal souls had no chance of doing it as well.
In time, the souls started to forget their identity, and most of them became demons. The rest became peaceful spirits, but people can taint those in time, thus the hurry to remove the Veil before all the spirits are changed for good, and the Corypheus debacle..
I don’t even think there ever was a huge trap setting to begin with. All he had to do was raise the Veil during the night, when most people were asleep, even the ‘gods’. Well, except the night guards, some people who got the night shift, bars, brothels, temples, stc. Maybe he even saved some in some way. He could just command all his followers to keep a three day vigil in honor of Mythal. Nobody would have tought ‘****! He’s blowing up the Fade’ with just that information.
Agreed, he once said that every elf was able to access the Fade at will, doesn’t mean they all did all the time. Maybe the slaves and the poor part of the society had limited or no access. After all, the big shots couldn’t feel so awesome while traipsing in the Fade if everybody could do it at will.
But of those that could access the Fade at night, I think many were doing it. After all, if you can choose between random dream no775/nightmare and a customizable dream in which you are trapped on an island with 100 naked supermodels, all madly in love with you and willing to satisfy your every whim, of course we’ll go for the nightmare.
After the Veil was created, the Fade was very quiet for a long time. It took a long time before a Tevinter dreamer(why a dreamer especially if every mage could access it?) managed to pierce the Veil and learned blood magic from a spirit/demon.
We also know from the Tevinter lore that at their peak the Tevinter Imperium was ruled by a group of strong dreamers(especially from the comics). So the old gods realized that in order to weaken the Veil they needed a lot of blood magic, a lot of massacres, and a lot of people in duress/pain. So enter the Blight, Circles, demonic possession, Veil is getting weaker with each Blight, each demon possession, each case of blood magic(the Contess).
Back on the topic of the People, we don’t know if Fen’harel sees even the demons as bad. They are his People after all, and they became demons because he trapped them in the Fade and couldn’t free them in time. So they probably rate higher on his like scale as any living being in the world. After all, he seems to have quite a nice chat with the demon we can feed Hawke to, and they seem to know each other quite well. He even admits that he has many friends among the spirits, while he sees the living as a bunch of tranquils. And let’s not forget how berserk he gets when some stupid mages summon a peaceful spirit, force him(her) do do something bad, thus turning it into a demon. Solas has to kill it after he fights it, and if the Inq allows it, he can slaughter all the mages that did that.
Since creating the Veil rifts had no adverse effect on spirits, removing the Veil will only benefit the inhabitants of the Fade, who will be able to walk the physical world at will.
On the other hand, the living, and I mean the one that survive the Veil being removed, will probably have a very nasty wake up call.
And at the end of Trespasser we see that elves start disappearing all over Thedas, so maybe he has a safe zone that won’t be affected as bad as the rest of the world, like the Crossroads or some underground temple.
Or maybe he just needs some suckers to use their blood to have the power to bring down the Veil and release the Kraken. Ahhh, I mean the spirits of the Fade.