Yes, he would have reacted differently. Anyone would have reacted differently. If someone saw someone else shooting a gun in the middle of a mall, they would surely back away. The only reason he was able to go up to her was probably because he thought she had a toy gun.
There is no way that you would react calmly in lieu of the constant violence going on today. I think being polite and trying to talk people down is over. Looking back in time i'm sure this man would have been much more aggressive in his stance, possibly knocking the perpetrator out cold. This is the 2016 me reacting to a mall shooting though. A lot has changed since 1985.
No, I don't believe John Laufer would have acted the same way had he known that Sylvia Seegrist's gun was real. I feel he would have hesitated, or he would have approached her in a much more stealth manner. He may have done the same thing, but the manner in which he did it would have been different.
No, I do not believe that John Laufer would have reacted the same way to Sylvia Seegrist if he had know that she had a real gun. There is a good chance that Laufer, however, thought the gun may have been real or at least threatening at the time, he just reacted without thinking about it.