It boils down to that.
Even if it turned out that the perp was not a threat, it would have been far far better to get Trump to safety first, then investigste the incident- better safe than Sorry, especially in light of the fact that there apparently was co corn that Iran wanted to try something too we are now learning. There was no excuse for not getting Trump off the stage as soon as they discovered someone acting very suspiciously.
The ss had to know the perp was a danger after they saw him point a gun at the cop and drive him off. But even being on that roof should have triggered a reaction by ss to cover Trump and get him to safety.