[Quoth]

You need the PIN, I think.

Parental mode on Kindles blocks all previously copied USB books. It's stupid.

You did the long 20 s factory reset?

The case opens easily (start top corner) and the battery (cell) is held on by 2 small screws. Don't lose the washers between PCB and battery!

That's the ultimate reset.

There is also an internal button, but I don't know what that does.

However it's crippled (no collections) without registration and that can't be done now. Even before it often didn't work.

Disconnecting the battery doesn't deregister, or didn't in the past, so it may not affect parental control, which certainly is partially controlled on Amazon's server.

I suspect Amazon don't care.