Same problem, though. Three entrances. No reason to expect Thompson at any of them, at any time (why not expect Thompson to be staying at the Hilton, Thompson not listed as a speaker, etc). I mean, I guess we can't rule out blind stupidity anymore, as he still had a gun and the tainted NJ ID on him. Without those, he might still have walked.
Eta: I can't stress this enough: the killer should have had no reason to expect Thompson to be at this conference, or if he was, to be staying at the Marriot instead of the Hilton or elsewhere, and consequently no reason to be anywhere we re near that door at any time, much less the perfect time. Unless he had reason to expect exactly that.