"1) It didn't happen because, well it couldn't. The mat is heavy and stiff enough (with all them little fingers on the underside to dig into the installed carpet) that we could not slide it forward. Besides the design of the mat fits around the foot rest which also blocks it movement."

I was actually able to replicate this problem this week. The mat itself is incappable to go over the gas pedal off course. This problem happen by a combination of two things, rubber mat missplacement and floored gas pedal. When the mat shifts its position from getting in and out of the car (notice it's half a persons body weight at least and moving sideways) it can go into a possition where if you depress the accelerator all the way, it will will hold the pedal stuck. A side movement pulling the pedal upwards with the foot should take care of this. Heck I even took pictures with my cell phone for I was able to replicate it three time on my 10 miles commute to work. Then again, P&G techniques develope sensitive feet!
I would suggest all drivers with all weather mats from Toyota to anchor them by using the holes both on the mats and the car. Just do it with something that can be easily removed for cleaning purposes.