> Will autopilot even let you floor the accelerator with a car right in front of you?

Yes. Autopilot does not override any user input. This is a major reason why L2 system failure will always be the responsibility of the driver.

Which would not result in an instant 0% -> 100% reading. One would expect a gradual slope from 0% to, say, 30%, then a sharp increase to 100% some tens or hundreds of milliseconds later.

Normally, from my understanding of these sorts of accidents, you wouldn't fail to move your foot over if you're only braking gently but you might if you need to slam down the brake. As your reflexes slow down as you age you'll find yourself getting into more situations where you need to slam the brake. And it will more often make sense to apply a hard brake now because you know you won't be able to increase the braking force in time later if it becomes necessary. I don't remember the last time I had to slam the brakes in my own driving but when I'm riding with my father he has to do it pretty regularly.

Nope. Unless they have really shitty logging intervals, which would also be a problem.