Sloppy? Well, sure... but if we take Cory's 'years in the making' dialogue to mean he's held the orb for years (I'm thinking since shortly after he woke, and I believe the devs have said Legacy is supposed to have occurred at some point between Acts 2 and 3... That's what - between 5 and 10 years before the Conclave? So it's possible Solas 'gave' him the orb before the Kirkwall Chantry is destroyed. At which point, i doubt anyone would have conceived of something like the Conclave - a giant gathering of very important people all in one place, as there wouldn't have been a need before the mage/templar war began. I imagine Solas figured in the unlikely even things didn't go as planned (and unlikely that would have seemed - Solas is the proud one, after all) what was the worst that could happen? A city gets wiped out? He might have see that as a tragic, but acceptable loss.
Yes, I assume everybody with a great vision would sacrifice a few people. But what was the plan? What are the reasons that made him decide the way he did?
So, Cory got the orb around Kirkwall Act 2. Why? Why did Solas give it away right after waking instead of taking his time to become powerful enough himself? He could at least have tried to power up before handing out a powerful artifact to anybody. He must have had a reason for that.
Then he let Cory run around with it for awhile, obviously without doing much himself. Why? Second dubious reason.
Then Cory went to the conclave, and obviously he needed the Divine for his plan. He must have panned something beforehand. You don't sit around idly for 4 years until suddenly there's an unexpected mage rebellion. Or at least I would find that very odd. So, between being freed, getting the orb, and the rebellion Cory must have done something, planned somehow. Also, his plan was rather extensive. It included the mages, the Templars, the Wardens, and the orb. Solas must have known. How come so many things surprised him throughout the game?
Seeing all the time that must have passed and that Solas surely observed Cory all the time I just find it so hard to believe he really only thought "He'll die, no problem". Sure, we can say that's what the Nightmare refers to: "Your pride will be the death of you". But I would just be very disappointed of somebody like Solas to make such decisions without a really good reason.