But they'll still release press releases saying "The vehicle had warned the driver of inattentiveness and to keep his hands on the wheel"... and oh-so-conveniently ignore "... once, fourteen minutes before the accident" (which, knowing their system now, means that was the last warning, and the attention system hadn't been tripped between then and the accident).

That's an interesting problem. The right answer mostly depends on the distribution of crashes at time t since deactivating autopilot. I would personally guess the relevance of autopilot fades to near 0 once you're 30 seconds since deactivation for 99.9% of crashes.

5 feels a little too aggressive, but would probably capture the majority of the true positives. I would have picked 10-15 seconds based on my gut.

That depends. If you're taking over from autopilot after several hours of passively resting behind the wheel, perhaps it will take you more than 30 seconds to accustom yourself to the task.

What situation could you possibly be in where its autopilots fault but takes more than 30 seconds to cause a problem AND it was necessary for you to take control?

Car steers onto opposite lane on interstate at night/no traffic?

You're not wrong, but to my knowledge, nothing like that has ever happened, and it would have been very newsworthy if it had, even absent of fatalities.