I think many would open the exit.
Not sure on who's wording it is but
...in the event of an accident AND only when instructed to do so.....
Firstly, the first bit is what sticks in your mind, not the bit after the AND.
Secondly, the first is irrelevant, better would be "If instructed by the crew to open the exit...."
I have a horrible feeling that when I am next to an over-wing exit I will probably have that door open and thrown out a.s.a.p. Whether I would actually get out is secondary.
Of course, I can't be sure, it has never happened, but I doubt I am an extreme case. I am wired to act in an emergency and 2000-3000 safety briefings is not likely to change that, though a few actual practice events probably would. You guys get to train this stuff, we do not. What if you were just told what to do in the case of an engine failure but never actually got to practice one?