Solas doesn't seem to have much of a head for thinking through possibilities. He went ahead and created the Veil, never apparently thinking it would do anything other than trap the other gods. Yet clearly this required more than the Veil because no one has ever encountered them there, unless of course the Black City is their prison. Then he arranged for Corypheus to take his orb. How did he know about Corypheus? He hadn't yet revealed himself to the world, just the Venatori. The very fact that he was an ancient Tevinter should have raised alarm bells in Solas' mind because that meant somehow he had survived through centuries from that time.
You raise the perfectly valid point, apart from the terrible idea of destroying the present, that if the Veil comes down then surely the bad gods he banished will return. "I have plans." I wish we could have replied something rather sarcastic at that point. Mind you he admits he has sacrificed so many people in one hopeless plan after another. Yet the penny still hasn't dropped. Your plans are always deeply flawed, Solas.
The thing is he does believe in discussing plans with other people, even if he doesn't think they might try to stop him. When he asks you what you are going to do with the Well, even if you didn't drink so actually the questions should be to Morrigan, if you reply something to the effect that you will take the advice of others, then you get "Solas Disapproves". Of course if you simply say, "I'll make the world a better place", he actually asks what if you wake up one day and find things are worse than before. You reply, "You keeping trying until you get it right". "Solas Approves". Solas only ever approves of what he wants to hear, thus reinforcing his own prejudices and deeply flawed plans for the world.
Solas gave his orb to Corypheus because it fitted with Solas' plans and he lacked the imagination to think it could be otherwise.