Some newish tidbits.
The shooter caught the attention of security when he showed up with rangefinder binoculars. Note those are legal. He evidently did not have the rifle at that point. As long as he was outside the inner perimeter, he could legally carry the rifle anyway
He got on the roof by climbing on the AC, not with a ladder.
Snipers near Trump were using binoculars watching him watching them at some point.
It looks like the failure, besides loose gun laws, was not including those buildings inside the inner perimeter controlled by the SS.