Yes I would be amazed if it took more than one shot from the pros and it should have happened in just a couple seconds. They just don't miss at ranges much under 2000 yards. They might have fired another round or two for security, common tactic at the pro level is one or two shots to center of mass, and one to the head. That way if the target is wearing a vest you at least stun/disrupt him to buy time, and the head shot is to make sure. But depends on the target aspect, if he was prone facing the sniper, maybe head shot to start with, since a near miss would likely hit center of mass anyway.

The only reason I can imagine for the delay was if they thought the guy might have been local PD. Apparently the building was the CP for the locals, so not utterly implausible that Barney Fife might have climbed up there for some reason. Especially if his clothes were similar to what the cops were wearing.

No excuse if that's what happened, USSS leadership shoud have clearly coordinated that, and everybody involved should have known: Either somebody will be posted on the roof, or it's a no-go area and thus weapons free.