My read of it is that until the target's weapon is leveled at a target, they aren't allowed to shoot unless they're using tasers or capsicum spray.
Action beats reaction every time. There is no way to get the first shot off if you wait for the other guy to move first. A person with a knife can clear 20' and stab you in the heart before you can draw and shoot them. The FBI proved this. Mythbusters did too.
Han didn't need to shoot under the table, he could have easily drew and shot Greedo before Greedo could even pull the trigger. Action beats reaction every time.
Is any Star Wars debate 6 degrees of Han Shot First?