He gave Corypheus the Orb because he was the only person with sufficient power to activate it, as the only other option was to ask Flemeth and Solas knew that she'd refuse outright to go along with the plan.
The only thing I don't get is why Solas seems to want to tear down the Veil at some times and not at others?
The Orb can be used to tear a hole in the Veil, which is the thing we see it being used for most often. Abelas' comments about how the Anchor being familar magic seems to imply that the ability to punch a hole into the Fade was something the Ancient Elves knew of, which seems consistent with the Eluvians and Crossroads showing that the Ancient Elves were capable of engineering reality itself to create pocket dimensions within the Fade? This would also likely be the way in which Fen'Harel sealed away the Elven Gods according to legend?
Futhermore, the Mask of Fen'Harel from Dragon Age: Redemption also was being used by Saarebas there to create a tear in the Fade? Although given that there were several quibbles with bits of the lore in that series, it's ambiguously canon?
Then you have that initial conversation with him in Haven about how the Veil seperating the Fade and Thedas isn't necessarily a good thing, making it sound like a world where reality is defined by imagination is something that he'd prefer?
However, why does Solas then proceed to help us seal the Breach, close Fade Rifts and also direct us to Elven Artefacts to shore up the Fade? If he wanted the Veil to be torn asunder and the two worlds to merge into one, why does he aid us in preventing that from happening?