Flinch, yes. People all around the area flinched and ducked. The person you mentioned ducked and possibly fell or possibly hit the ground.
I've seen footage of the dust or whatever from the railing. So what it went sideways? I don't find John Cullen convincing. He did an interview with someone on Youtube showing clips of lots of people on the back row of the bleacher where the person was hit suggesting they all showed signs of being hit/clipped - indicating the shot travelled along the back row of the bleacher. He gave no evidence that they were hit and didn't seem to consider that they were reacting to the sound of the shot(s) instead.
Yes, have seen the bodycam video.
His recent ones on the story of the ladders and the clips of Crooks running on the roof are excellent. Includes some footage of the water tower around the time of the shooting and also shows Police cars parked up in that area. No shooter to be seen there.
He also said the snipers were taking fire, which has been claimed by someone as 'direct from the source'. A little further on from 15:30 in the video you linked, you can see that the sniper ducks just before the first shot rings out (edit: this might be just after the first shot). A few seconds later he and his mate re-align their rifles towards the trees, and this was also mentioned by one lady commentator in some other footage.
So Cullen does have a point about a possible shot fired from the trees.
I don't see any evidence of someone on the water tower either.
One eye witness, who mentions the water tower, also said someone she spoke to had been sent photos of two dead people - not Crooks - on his phone.
So there's other information that needs explaining, whether it indicates only one or more than one shooter.
Yes the footage of Crooks running on the roof certainly shows how it would have been difficult to stop him. Would be interesting to know why they didn't shoot him before he fired.
The photos of the window are significant, if you understand that they show the same window which was directly in line with Trump and Crooks, and noting the time at which it suddenly looked different, seconds before the shots rang out.