First, I haven't seen it substantiated anywhere that protesters suffered tear gas, flashbags, rubber bullets, or any of these other riot control measures. That isn't in the video, and I don't find some random tweet a credible source.
Second, you are deliberately misrepresenting the nature of this hypothetical. Trump initiates a Presidential train to move across the street. SS agents would fan out and clear the way. They would first ask people to move. If those people didn't move, they would then issue a lawful order according to their authority for those people to move in the interest of the security of the President. The SS is in charge of his security, and the judges of what is an acceptable distance for standerbys to be to the route he intends to travel.
Imagine if this were Obama. I don't care about hypotheticals about Obama not doing this. Imagine it were him. If Obama wanted to walk this route, he'd walk this route. He is the President. You accede to that authority.