1. Itachi was directing it to follow Sasuke, he didn't attempt to simply spawn it on him, because he wasn't trying to kill him.
2. You have no way of knowing what's hyperbole or not. It managed to burn through the flesh of a creature that spits fire, and a cement wall in literally seconds. Either way, it doesn't even matter.
3. Kagutsuchi is not instant. The spawning of amaterasu is. Amaterasu spawned within a solid surface, through physical obstacles between the eye and the target. That's the point that is being made when people say amaterasu is instant. It doesn't have to travel through the aether as a projectile. Meaning, to dodge it, you have to be out of sight when the technique is activated, or you will get hit.