In your option on a scale of 1 to 10 - how badly did the Secret Service screw up?
I'm not a gun guy and I've been lazy (I only read the very short article) but to me it seems like a 10/10 "What the fuck were they thinking?" screw up.
Sniper shot at former President Donald Trump from 130 yards away on roof of manufacturing plant
An AR-15 would be perfectly fine at 130 yards. Without even any fancy optics or scopes or anything, just iron sights.
To your other points, the shooter didn't fire many rounds, I'm going to guess the rifle was an SKS.