Just noting for posterity that this ought to be reproducible. On a real NES-101, I was able to trigger it nearly every time I entered that room, by pacing back and forth repeatedly in the corners. Then I forgot about it because I was trying an ironman win and it seemed cheap. :7
(Sorry for bumping without even attempting a test movie, but compiling any emu on this ancient computer is baffling me currently. HTH.)
The 2600's difficulty settings were physical switches on the outside of the console deck, so yes, they could be changed at any time. A few games even used them as additional controls, e.g. Space Shuttle.