I had a thought.
If Solas is a Dreamer, then why did he not use his Dreamer powers to locate Corypheus and either kill him or direct the Inquisition's forces to him or at the very least, alert his agents to where Corypheus was so that they can steal his Orb ? I mean, Corypheus must have used the Fade a lot and Dreamers are capable of entering the Fade and killing people right ? Okay so perhaps Corypheus does not sleep. But what about Samson or Calpernia ? They sure needed sleep. Furthermore, Calpernia is a mage and the Venatori that Corypheus are allied with have mages with them. We know that Solas killed Felassan in his dreams so what stopped him from doing this ?
It's hard to say how much Solas' abilities as a dreamer were diminished by events like the Breach - it may also be that invading or locating someone's mind is trickier than we think it is.
After all - if he knew the mind of Corypheus, would he ever give him the orb and assume that it'll kill him?
And it's not like Cole can give us precise reading where Cory is. In fact when we ask him for a reading on Corypheus, he says that Cory is "too loud" and his thoughts are all jumbled, so he can only get partial reading.
There's also another thing - perhaps Solas doesn't want to touch blighted minds (or minds close to the Blight). He's freaked out by the Blight as it is.
Plus, don't forget that Venatori mages use A LOT of blood magic, and blood magic - as Solas himself tells us - makes it harder for a mage to cross through the Fade, which may also mean that they're harder to localize by spirits/dreamers.