If it isn't letting you trim back 1 frame, my guess would be that our material has nothing left to trim with. There is no rule saying you can't trim back 1 frame. type -1 (or is it 1-?), or simply press the j key to trim backwards in real time.
As a side note, when I'm zoomed a bit further out on my timeline, and I notice that there is a flash edit like you have here, I usually just type -10. The trim point will always stop at the next edit, so you won't overwrite anything from the next previous shot. And if (while zoomed out) I mistook a 2 frame flash for a 1 frame flash, the -10 will take care of both frames.