1) Unless they are bastid tight, clipless pedals unclip rather easily against anything other than normal pedalling up-and-down forces. Given that clipless pedals are like small ski bindings, which (hopefully) undo if you fall over 'wrong', you have to be doing a fairly dead-on straight OTB or falling off the back to not be tugging your cleats out of the pedals at a sufficient angle for them to disengage with a realitively low force.
2) I fell over clipped in twice in 2001 when I first got clipless pedals. I have come of plenty of times since then and never been attached to the pedals at the end of it. I don't know if the fraction of a second longer it must take to twist out of the pedal was a factor in these crashes ie whether I could have ridden out of them on flats, but I can't remember even thinking about unclipping for many years, whether that is stopping at the lights, or suddenly having to jab a foot out on some really tricky trail, it just happens, even when the bike stalls and is at a virtual standstill.
3) Steve Peat and Greg Minnaar. I bet they can undclip quickly at high speed.